duplicate entries appear in cpanfile.snapshot

[miyagawa]

When upgrading/downgrading a module in transient dependency, there're cases where the module appears twice in the snapshot file. Usually when you run carmel install again it will disappear.

➜  cat cpanfile
requires 'Plack';
➜  carmel install
(snip)
➜  grep URI- cpanfile.snapshot 
  URI-5.10
    pathname: O/OA/OALDERS/URI-5.10.tar.gz

➜  cat cpanfile
requires 'Plack';
requires 'URI', '==5.09';
➜  carmel install
(snip)
➜  grep URI- cpanfile.snapshot
  URI-5.09
    pathname: O/OA/OALDERS/URI-5.09.tar.gz
  URI-5.10
    pathname: O/OA/OALDERS/URI-5.10.tar.gz

With -v:

URI-5.10 found in snapshot for URI
URI not found in snapshot
   URI (5.08) in /Users/miyagawa/.carmel/5.32.1-darwin-2level/builds/URI-5.08
URI-5.10 found in snapshot for URI::Escape
URI::Escape not found in snapshot
   URI::Escape (5.10) in /Users/miyagawa/.carmel/5.32.1-darwin-2level/builds/URI-5.10