Allow triangular gitlab-ci imports

Problem to solve

# base.yml
.some-step:
   script:
    - echo "something"

# file1.yml
include:
   local: base.yml

# file2.yml
include:
   local: base.yml

Then including both file1.yml and file2.yml like so:

include:
   - local: file1.yml
   - local: file2.yml

Will throw the following error:

Include `{local: base.yml}` was already included!

This appears to be thrown from this code

This is quite frustrating if you're building a semi-complex pipeline that includes some kind of common utilities, which might be included from multiple other files.

Proposal

Do not throw an error in this case. Am I missing something, or is this redundant? What would be the problem with ignoring a duplicate include like this?

Links / references