Deduping test email is case sensitive
Tested on 4.6.38 and 5.6.1.
When composing a mailing, the email address used in the "send test email to" field at the bottom is deduped. This is case sensitive, so fails to match emails that are the same but have different upper/lower case combinations. In this case, it will create a new contact with this interesting property: the value in the "email" field shows with the email address flattened to all lowercase, but the display name for the contact shows with the upper/lower case mix used when it was entered to test the mailing.
The upshot of all this is that if I'm putting together a mailing and send 6 test to the same email address (with an uppercase letter), I get 6 new duplicate contacts because it fails to match it each time.
Request to make the dedupe test for this mailing field case insensitive, since in general civi seems to want email addresses to be lowercase when they're used.
I can potentially make this change if someone can point me in the right direction.