url: don't send duplicate messages where more than one url with the same title is in a message

[RhinosF1]

The problem

Url can be spammy

The solution

If a message includes more than one url with the same title, only send it once.

Alternatives

I also considered ..., but this is the best way because ...

Notes

https://github.com/sopel-irc/sopel/blob/master/sopel/modules/url.py#L219 should store message in an array then clear the array of duplicates and then send it.