diff --git a/lib/bundler/resolver.rb b/lib/bundler/resolver.rb
index 0cf2da007aa..8794b6909b6 100644
--- a/lib/bundler/resolver.rb
+++ b/lib/bundler/resolver.rb
@@ -43,9 +43,12 @@ def initialize(index, source_requirements, base, gem_version_promoter, additiona
     def start(requirements)
       verify_gemfile_dependencies_are_found!(requirements)
       dg = @resolver.resolve(requirements, @base_dg)
-      dg.map(&:payload).
+      dg.
+        tap {|resolved| validate_resolved_specs!(resolved) }.
+        map(&:payload).
         reject {|sg| sg.name.end_with?("\0") }.
-        map(&:to_specs).flatten
+        map(&:to_specs).
+        flatten
     rescue Molinillo::VersionConflict => e
       message = version_conflict_message(e)
       raise VersionConflict.new(e.conflicts.keys.uniq, message)
@@ -353,5 +356,29 @@ def version_conflict_message(e)
         :version_for_spec => lambda {|spec| spec.version }
       )
     end
+
+    def validate_resolved_specs!(resolved_specs)
+      resolved_specs.each do |v|
+        name = v.name
+        next unless sources = relevant_sources_for_vertex(v)
+        sources.compact!
+        if default_index = sources.index(@source_requirements[:default])
+          sources.delete_at(default_index)
+        end
+        sources.reject! {|s| s.specs[name].empty? }
+        sources.uniq!
+        next if sources.size <= 1
+
+        multisource_disabled = Bundler.feature_flag.disable_multisource?
+
+        msg = ["The gem '#{name}' was found in multiple relevant sources."]
+        msg.concat sources.map {|s| "  * #{s}" }.sort
+        msg << "You #{multisource_disabled ? :must : :should} add this gem to the source block for the source you wish it to be installed from."
+        msg = msg.join("\n")
+
+        raise SecurityError, msg if multisource_disabled
+        Bundler.ui.error "Warning: #{msg}"
+      end
+    end
   end
 end
diff --git a/lib/bundler/version.rb b/lib/bundler/version.rb
index 424965771a4..e8a180f5db7 100644
--- a/lib/bundler/version.rb
+++ b/lib/bundler/version.rb
@@ -7,7 +7,7 @@ module Bundler
   # We're doing this because we might write tests that deal
   # with other versions of bundler and we are unsure how to
   # handle this better.
-  VERSION = "1.15.4" unless defined?(::Bundler::VERSION)
+  VERSION = "2.15.4" unless defined?(::Bundler::VERSION)
 
   def self.overwrite_loaded_gem_version
     begin
diff --git a/spec/install/gemfile/sources_spec.rb b/spec/install/gemfile/sources_spec.rb
index 0b837f87a10..8d69a663de5 100644
--- a/spec/install/gemfile/sources_spec.rb
+++ b/spec/install/gemfile/sources_spec.rb
@@ -545,7 +545,7 @@
       end
 
       it "does not re-resolve" do
-        bundle :install, :verbose => true
+        bundle! :install, :verbose => true
         expect(out).to include("using resolution from the lockfile")
         expect(out).not_to include("re-resolving dependencies")
       end
@@ -617,4 +617,31 @@
       end
     end
   end
+
+  context "when a gem is available from multiple ambiguous sources", :bundler => "2" do
+    it "raises, suggesting a source block" do
+      build_repo4 do
+        build_gem "depends_on_rack" do |s|
+          s.add_dependency "rack"
+        end
+        build_gem "rack"
+      end
+
+      install_gemfile <<-G
+        source "file:#{gem_repo4}"
+        source "file:#{gem_repo1}" do
+          gem "thin"
+        end
+        gem "depends_on_rack"
+      G
+      expect(last_command).to be_failure
+      expect(last_command.stderr).to eq strip_whitespace(<<-EOS).strip
+        The gem 'rack' was found in multiple relevant sources.
+          * rubygems repository file:#{gem_repo1}/ or installed locally
+          * rubygems repository file:#{gem_repo4}/ or installed locally
+        You must add this gem to the source block for the source you wish it to be installed from.
+      EOS
+      expect(the_bundle).not_to be_locked
+    end
+  end
 end