Silently ignore duplicate heap entries

Vendor blobs on ryu mprotect heap pages, causing a single chunk mapping
to appear as multiple mappings.  The heap iterator has to expand the
requested range to cover the beginning of the chunk to find the chunk
metadata, which will lead to duplicate identical allocations being
reported from iterating over each of the split mappings.  Silently
ignore identical allocations, and only warn on non-identical allocations
that overlap.

Bug: 28269332
Change-Id: Ied2ab9270f65d00a887c7ce1a93fbf0617d69be0
(cherry picked from commit cecd6401)