Allow duplicate files inputs in soong_zip

Accept duplicate file inputs in soong_zip when they are the same
source file.  This came up when trying to zip lint srcs, as some
java modules have duplicate source files that seem to be ignored
by javac.

Test: TestZip
Bug: 216456886
Change-Id: I8c43df9aded8cf094afaed79cca2b9eb091cc861