Version

2.5.2

Reproduction link

https://jsfiddle.net/50wL7mdz/71920/

Steps to reproduce

run the jsfiddle program and look at the textarea content, more precisely the last "domProps"

What is expected?

do not generate same key twice

What is actually happening?

... .domProps:{"value":'foo',"value":(model)} ...

This will rise an error in strict mode.
The JSFiddle example don't rise the error because it not in strict mode.
I have discovered the issue using webpack + babel + test on IE11