The Statistics page should display redirected templates in bright red, and instead of Edit, should display Redirected with the following popup error message:
I am sorry but (template1) has been redirected to (template2) in Wikipedia, as you can verify http://XX.wikipedia.org/w/index.php?title=(template1)&redirect=no "here". You need to "migrate" this mapping to (template2) (find some discriminator field to distinguish between the two uses) and then delete this mapping.
the validator should display the same error If a mapping uses a redirected template
I am sorry but (template1) has been redirected to (template2) in Wikipedia, as you can verify http://XX.wikipedia.org/w/index.php?title=(template1)&redirect=no "here". You need to "migrate" this mapping to (template2) (find some discriminator field to distinguish between the two uses) and then delete this mapping.
Example for testing: