Filter out duplicated targets in function get_iter

[dachengx]

What is the problem / what does the code in this PR do

Fix #846

The reason this can work is explained here: #859 (comment)

Can you briefly describe how it works?

Remove duplicated targets when get_iter.

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• Update the docstring(s)
• Update the documentation
• Tests to check the (new) code is working as desired.
• Does it solve one of the open issues on github?

Please make sure that all automated tests have passed before asking for a review (you can save the PR as a draft otherwise).

[coveralls]

coverage: 90.43% (+0.001%) from 90.429%
when pulling 187970f on unique_targets
into 2218e78 on master.