Add a test that properly verifies the handling of the duplicated context when caching Uni

This is related to the commit f781b7fd106fd2d90656ac78d21f8e01374f0b68.
Tests added in f5af6b40e34ecdf7bd7596d4806b5866af02b38f were not reproducing the issue.
I kept these tests because they do test a few other things (but not the original issue).