Do not emit duplicate string for pragma([lib|linkerDirective])

[thewilsonator]

No description provided.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19329	normal	pragms(lib) and pragma(linkerDirective) can emit duplicate entries to the object

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#16849"

[thewilsonator]

cc @TurkeyMan for the tests here