Add warnings when doppelganger CLs are found.
Add warnings when doppelganger CLs are found.

When two CLs have the same Change-ID, this can sometimes cause issues
for the Commit Queue, because it uses the Change-ID as a lookup key in
its internal dictionaries.

Note that the commit queue does not use Change-IDs to look up
dependencies anymore, so most of the issues originally addressed by
parent lookup are mostly gone now.

BUG=none
TEST=All unit tests.

Change-Id: I8560cca433a6439d75256d5ce48187e1886d538f
Reviewed-on: https://gerrit.chromium.org/gerrit/57265
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite