New resolver - duplicate requirement check

[pfmoore]

Pip currently checks for multiple requirements for the same project:

>pip install "django>2.0" "django<1.11"
ERROR: Double requirement given: django<1.11 (already in django>2.0, name='django')

That happens even with the new resolver.

Question - do we want to retain this check with the new resolver, or is it unnecessary? Certainly the new resolver will be capable of handling double requirements (in ghe above case, giving an error because they conflict, in other cases where they don't, finding a solution that satisfies both).

I haven't yet checked, but I assume this check is only done on "top level" requirements, and multiple dependencies on the same project are fine.

@pradyunsg @uranusjr @ei8fdb

[uranusjr]

To add some additional context (since the above example does fail, and might give an impression the check is a good thing). The problem is that the current implementation is extremely limiting. One common frustration is when you need different versions of a package in different environments:

# base.txt
Django>=3

# prod.txt
-r base.txt

# I want to try upgrading the code base to 3.1, but let’s be safe in production.
Django==3.0

And this would fail the check:

$ pip install -r prod.txt
ERROR: Double requirement given: Django==3.0 (already in Django>=3, name='Django')

So I would say this check should go eventually. The new resolver already eliminates most of the usages (it’s in RequirementSet.add_requirement()), but it is still called right now during the initial RequirementSet construction (before it reaches the resolver). See #7571.

[uranusjr]

Tracking this in #7571 instead.