Restore in-memory check for duplicate layers

[mtrmac]

… per #63 (comment) . That PR did add an imageCopier.copiedBlobs but it has no uses.

[rhatdan]

@mtrmac @vrothberg What is the state of this issue. It is two years old, can we close it?

[vrothberg]

AFAIU the issue, we can close it.

[mtrmac]

(With #611, alternatively:

• an in-memory duplicate check exists in the current version.
• just silently skipping blob copies in generic code is no longer permissible; the code should involve TryReusingBlob even for duplicates.

See #611 (comment) )