Downgrade another duplicate method DCHECK to WARNING

This DCHECK got missed when writing change
'Ib26f30718726a9064056855192cef302f3df1eeb'. This is also caused by
duplicate methods in dex files causing some ArtMethod's to appear in
vtables more than once.

Bug: 32549051
Bug: 32546154
Bug: 36446088
Bug: 36399552

Test: ./test/testrunner/testrunner.py --host -j40
Change-Id: Id8ef07d7185958a91f008c2297db96c090dbecd7