In the current implementation, an error is raised if a remote with the
same name is defined more than once. The check is only that the remote
has the same name as an existing remote.
With the support for multiple local manifests, it is more likely than
before that the same remote is defined in more than one manifest.
Change the check so that it only raises an error if a remote is defined
more than once with the same name, but different attributes.
Change-Id: Ic3608646cf9f40aa2bea7015d3ecd099c5f5f835