allow to use non-unique index ids

• added a new index and pointed it at one nexus group. The context provided by nexus is "public-merged".
• then edited the existing Central index url to point at another nexus instance.
• clicked retrieve context and it also said public-merged.
• When i saved it, the central one simply disappeared.

What if i really want both indexes? I don't have control all the time of the context used on the remote url so m2e should be able to handle this.

Nexus deals with it by changing the context after it's downloaded before attempting to merge them together.