diff --git a/CHANGELOG.md b/CHANGELOG.md
index eae0bf86e..915a02137 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,9 @@
 
 ## [Unreleased]
 
+- Added a warning if the Python buildpack has been run multiple times in the same build. In January 2025 this warning will be made an error. ([#1724](https://github.com/heroku/heroku-buildpack-python/pull/1724))
+- Added a warning if an existing `.heroku/python/` directory is found in the app source. In January 2025 this warning will be made an error. ([#1724](https://github.com/heroku/heroku-buildpack-python/pull/1724))
+- Improved the error message shown if the buildpack is used on an unsupported stack. ([#1724](https://github.com/heroku/heroku-buildpack-python/pull/1724))
 - Fixed Dev Center links to reflect recent article URL changes. ([#1723](https://github.com/heroku/heroku-buildpack-python/pull/1723))
 
 ## [v271] - 2024-12-12
diff --git a/bin/compile b/bin/compile
index 495d8c20d..7edf8a44c 100755
--- a/bin/compile
+++ b/bin/compile
@@ -21,6 +21,7 @@ BUILDPACK_DIR=$(cd "$(dirname "$(dirname "${BASH_SOURCE[0]}")")" && pwd)
 source "${BUILDPACK_DIR}/bin/utils"
 source "${BUILDPACK_DIR}/lib/utils.sh"
 source "${BUILDPACK_DIR}/lib/cache.sh"
+source "${BUILDPACK_DIR}/lib/checks.sh"
 source "${BUILDPACK_DIR}/lib/hooks.sh"
 source "${BUILDPACK_DIR}/lib/metadata.sh"
 source "${BUILDPACK_DIR}/lib/output.sh"
@@ -32,10 +33,15 @@ source "${BUILDPACK_DIR}/lib/poetry.sh"
 
 compile_start_time=$(nowms)
 
-# Initialise metadata store.
+# Initialise the buildpack metadata store.
+# This is used to track state across builds (for cache invalidation and messaging when build
+# configuration changes) and also so that `bin/report` can generate the build report.
 meta_init "${CACHE_DIR}" "python"
 meta_setup
 
+checks::ensure_supported_stack "${STACK:?Required env var STACK is not set}"
+checks::warn_if_duplicate_python_buildpack "${BUILD_DIR}"
+
 # Prepend proper path for old-school virtualenv hackery.
 # This may not be necessary.
 export PATH=:/usr/local/bin:$PATH
@@ -103,6 +109,10 @@ cd "$BUILD_DIR"
 # Runs a `bin/pre_compile` script if found in the app source, allowing build customisation.
 hooks::run_hook "pre_compile"
 
+# This check must be after the pre_compile hook, so that we can check not only the original
+# app source, but also that the hook hasn't written to '.heroku/python/' either.
+checks::warn_if_existing_python_dir_present "${BUILD_DIR}"
+
 package_manager="$(package_manager::determine_package_manager "${BUILD_DIR}")"
 meta_set "package_manager" "${package_manager}"
 
@@ -135,7 +145,7 @@ python_major_version="${python_full_version%.*}"
 meta_set "python_version" "${python_full_version}"
 meta_set "python_version_major" "${python_major_version}"
 
-cache::restore "${BUILD_DIR}" "${CACHE_DIR}" "${STACK:?}" "${cached_python_full_version}" "${python_full_version}" "${package_manager}"
+cache::restore "${BUILD_DIR}" "${CACHE_DIR}" "${STACK}" "${cached_python_full_version}" "${python_full_version}" "${package_manager}"
 
 # The directory for the .profile.d scripts.
 mkdir -p "$(dirname "$PROFILE_PATH")"
diff --git a/bin/report b/bin/report
index dcba72ef2..31045de8a 100755
--- a/bin/report
+++ b/bin/report
@@ -82,6 +82,8 @@ ALL_OTHER_FIELDS=(
 	cache_save_duration
 	dependencies_install_duration
 	django_collectstatic_duration
+	duplicate_python_buildpack
+	existing_python_dir
 	nltk_downloader_duration
 	package_manager_install_duration
 	pipenv_has_lockfile
diff --git a/lib/cache.sh b/lib/cache.sh
index 9d9415ba3..ef62d582f 100644
--- a/lib/cache.sh
+++ b/lib/cache.sh
@@ -138,7 +138,7 @@ function cache::restore() {
 		mkdir -p "${build_dir}/.heroku"
 
 		# NB: For now this has to handle files already existing in build_dir since some apps accidentally
-		# run the Python buildpack twice. TODO: Add an explicit check/error for duplicate buildpacks.
+		# run the Python buildpack twice. TODO: Refactor this once duplicate buildpacks become an error.
 		# TODO: Investigate why errors are ignored and ideally stop doing so.
 		# TODO: Compare the performance of moving the directory vs copying files.
 		cp -R "${cache_dir}/.heroku/python" "${build_dir}/.heroku/" &>/dev/null || true
diff --git a/lib/checks.sh b/lib/checks.sh
new file mode 100644
index 000000000..700a87f02
--- /dev/null
+++ b/lib/checks.sh
@@ -0,0 +1,116 @@
+#!/usr/bin/env bash
+
+function checks::ensure_supported_stack() {
+	local stack="${1}"
+
+	case "${stack}" in
+		heroku-20 | heroku-22 | heroku-24)
+			return 0
+			;;
+		cedar* | heroku-16 | heroku-18)
+			# This error will only ever be seen on non-Heroku environments, since the
+			# Heroku build system rejects builds using EOL stacks.
+			output::error <<-EOF
+				Error: The '${stack}' stack is no longer supported.
+
+				This buildpack no longer supports the '${stack}' stack since it has
+				reached its end-of-life:
+				https://devcenter.heroku.com/articles/stack#stack-support-details-for-apps-using-classic-buildpacks
+
+				Upgrade to a newer stack to continue using this buildpack.
+			EOF
+			meta_set "failure_reason" "stack::eol"
+			exit 1
+			;;
+		*)
+			output::error <<-EOF
+				Error: The '${stack}' stack is not recognised.
+
+				This buildpack does not recognise or support the '${stack}' stack.
+
+				If '${stack}' is a valid stack, make sure that you are using the latest
+				version of this buildpack and have not pinned to an older release:
+				https://devcenter.heroku.com/articles/managing-buildpacks#view-your-buildpacks
+				https://devcenter.heroku.com/articles/managing-buildpacks#classic-buildpacks-references
+			EOF
+			meta_set "failure_reason" "stack::unknown"
+			exit 1
+			;;
+	esac
+}
+
+# TODO: Turn this into an error in January 2025.
+function checks::warn_if_duplicate_python_buildpack() {
+	local build_dir="${1}"
+
+	# The check for the `PYTHONHOME` env var prevents this warning triggering in the case
+	# where the Python install was committed to the Git repo (which will be handled later).
+	# (The env var can only have come from the `export` file of an earlier buildpack,
+	# since app provided config vars haven't been exported to the environment here.)
+	if [[ -f "${build_dir}/.heroku/python/bin/python" && -v PYTHONHOME ]]; then
+		output::warning <<-EOF
+			Warning: The Python buildpack has already been run this build.
+
+			An existing Python installation was found in the build directory
+			from a buildpack run earlier in the build.
+
+			This normally means there are duplicate Python buildpacks set
+			on your on your app, which is not supported since it can cause
+			unexpected errors and slow down builds.
+
+			Check the buildpacks set on your app and remove any duplicate
+			Python buildpack entries:
+			https://devcenter.heroku.com/articles/managing-buildpacks#view-your-buildpacks
+			https://devcenter.heroku.com/articles/managing-buildpacks#remove-classic-buildpacks
+
+			If you have a use-case that requires duplicate buildpacks,
+			please comment on:
+			https://github.com/heroku/heroku-buildpack-python/issues/1704
+
+			In January 2025 this warning will be made an error.
+		EOF
+		meta_set "duplicate_python_buildpack" "true"
+		# shellcheck disable=SC2034 # This is used below until we make this check an error.
+		DUPLICATE_PYTHON_BUILDPACK=1
+	fi
+}
+
+# TODO: Turn this into an error in January 2025.
+function checks::warn_if_existing_python_dir_present() {
+	local build_dir="${1}"
+
+	# Avoid warning twice in the case of duplicate buildpacks.
+	# TODO: Remove this once `warn_if_duplicate_python_buildpack` becomes an error.
+	if [[ -v DUPLICATE_PYTHON_BUILDPACK ]]; then
+		return 0
+	fi
+
+	# We use `-e` here to catch the case where `python` is a file rather than a directory.
+	if [[ -e "${build_dir}/.heroku/python" ]]; then
+		output::warning <<-EOF
+			Warning: Existing '.heroku/python/' directory found.
+
+			Your app's source code contains an existing directory named
+			'.heroku/python/', which is where the Python buildpack needs
+			to install its files. This existing directory contains:
+
+			$(find .heroku/python/ -maxdepth 2 || true)
+
+			Writing to internal locations used by the Python buildpack
+			is not supported and can cause unexpected errors.
+
+			If you have committed a '.heroku/python/' directory to your
+			Git repo, you must delete it or use a different location.
+
+			Otherwise, check that an earlier buildpack or 'bin/pre_compile'
+			hook has not created this directory.
+
+			If you have a use-case that requires writing to this location,
+			please comment on:
+			https://github.com/heroku/heroku-buildpack-python/issues/1704
+
+			In January 2025 this warning will be made an error.
+		EOF
+		meta_set "existing_python_dir" "true"
+	fi
+}
diff --git a/spec/hatchet/checks_spec.rb b/spec/hatchet/checks_spec.rb
index d29554821..102f3e34c 100644
--- a/spec/hatchet/checks_spec.rb
+++ b/spec/hatchet/checks_spec.rb
@@ -3,6 +3,43 @@
 require_relative '../spec_helper'
 
 RSpec.describe 'Buildpack validation checks' do
+  context 'when there are duplicate Python buildpacks set on the app' do
+    let(:buildpacks) { %i[default default] }
+    let(:app) { Hatchet::Runner.new("spec/fixtures/python_#{DEFAULT_PYTHON_MAJOR_VERSION}", buildpacks:) }
+
+    it 'fails detection' do
+      app.deploy do |app|
+        expect(clean_output(app.output)).to include(<<~OUTPUT)
+          remote: -----> Python app detected
+          remote: 
+          remote:  !     Warning: The Python buildpack has already been run this build.
+          remote:  !     
+          remote:  !     An existing Python installation was found in the build directory
+          remote:  !     from a buildpack run earlier in the build.
+          remote:  !     
+          remote:  !     This normally means there are duplicate Python buildpacks set
+          remote:  !     on your on your app, which is not supported since it can cause
+          remote:  !     unexpected errors and slow down builds.
+          remote:  !     
+          remote:  !     Check the buildpacks set on your app and remove any duplicate
+          remote:  !     Python buildpack entries:
+          remote:  !     https://devcenter.heroku.com/articles/managing-buildpacks#view-your-buildpacks
+          remote:  !     https://devcenter.heroku.com/articles/managing-buildpacks#remove-classic-buildpacks
+          remote:  !     
+          remote:  !     If you have a use-case that requires duplicate buildpacks,
+          remote:  !     please comment on:
+          remote:  !     https://github.com/heroku/heroku-buildpack-python/issues/1704
+          remote:  !     
+          remote:  !     In January 2025 this warning will be made an error.
+          remote: 
+          remote: -----> Using Python #{DEFAULT_PYTHON_MAJOR_VERSION} specified in .python-version
+          remote: -----> Restoring cache
+          remote: -----> Using cached install of Python #{DEFAULT_PYTHON_FULL_VERSION}
+        OUTPUT
+      end
+    end
+  end
+
   context 'when the app source contains a broken Python install' do
     let(:app) { Hatchet::Runner.new('spec/fixtures/python_in_app_source', allow_failure: true) }
 
@@ -10,6 +47,32 @@
       app.deploy do |app|
         expect(clean_output(app.output)).to include(<<~OUTPUT)
           remote: -----> Python app detected
+          remote: 
+          remote:  !     Warning: Existing '.heroku/python/' directory found.
+          remote:  !     
+          remote:  !     Your app's source code contains an existing directory named
+          remote:  !     '.heroku/python/', which is where the Python buildpack needs
+          remote:  !     to install its files. This existing directory contains:
+          remote:  !     
+          remote:  !     .heroku/python/
+          remote:  !     .heroku/python/bin
+          remote:  !     .heroku/python/bin/python
+          remote:  !     
+          remote:  !     Writing to internal locations used by the Python buildpack
+          remote:  !     is not supported and can cause unexpected errors.
+          remote:  !     
+          remote:  !     If you have committed a '.heroku/python/' directory to your
+          remote:  !     Git repo, you must delete it or use a different location.
+          remote:  !     
+          remote:  !     Otherwise, check that an earlier buildpack or 'bin/pre_compile'
+          remote:  !     hook has not created this directory.
+          remote:  !     
+          remote:  !     If you have a use-case that requires writing to this location,
+          remote:  !     please comment on:
+          remote:  !     https://github.com/heroku/heroku-buildpack-python/issues/1704
+          remote:  !     
+          remote:  !     In January 2025 this warning will be made an error.
+          remote: 
           remote: -----> Using Python #{DEFAULT_PYTHON_MAJOR_VERSION} specified in .python-version
           remote: -----> Using cached install of Python #{DEFAULT_PYTHON_FULL_VERSION}
           remote: