A warning should appear if multiple recipients have the same address

[johnny9]

Originally posted by @rabbitholiness in #450 (comment)

If I use the same address for different recipients, then we should probably highlight that somehow. Based on how difficult it is to implement, there is a more generic way to just say "You're reusing this address for multiple recipients." But ideally we would say "This address is being reused for Recipient x and Recipient y." The names of the recipients could be a link to the corresponding form. Here's how that could look like.