Clarify behavior of duplicate endpoint URLs

In https://crbug.com/983000, we're considering what to do when two endpoints in the same endpoint group share the same URL, particularly if they have different weights and/or priorities. The spec should be more explicit about what to do in this case.

Consider the following example:

{'endpoints': [{'url': 'https://endpoint', 'weight': 1},{'url': 'https://different_endpoint','weight':1},{'url': 'https://endpoint', 'weight': 1}]}

We have at least four options:

1. Treat each endpoint separately, even if they share the same URL. If we do that, 'https://endpoint' gets a total weight of 2. It's unclear what this means for backoff logic; do we have separate backoff logic for each endpoint even if they have the same URL?
2. Merge endpoints that share the same URL. It's not clear how to merge two endpoints with different priorities, though, so I think keep them separate is probably better.
3. Silently ignore one of the endpoints with a duplicate URL. (Which one? What if they have different weight or priority?)
4. Reject the entire header. This feels unfriendly.