Don't call export_symbols.py with duplicate libs

Summary:
export_symbols.py discards duplicate symbols, assuming they have public definitions, so if we end
up calling it with duplicate libraries we will end up with an inaccurate export list.

Reviewers: jasonliu, stevewan, john.brawn

Reviewed By: john.brawn

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70918