This is called a nonlinear pattern. There have been several threads on the haskell-cafe mailing list about this, not long ago. Here are two:
http://www.mail-archive.com/haskell-cafe@haskell.org/msg59617.html
http://www.mail-archive.com/haskell-cafe@haskell.org/msg62491.html
Bottom line: it’s not impossible to implement, but was decided against for sake of simplicity.
By the way, you do not need if
or case
to work around this; the (slightly) cleaner way is to use a guard:
a `plus` b
| a == b = 2*a
| otherwise = a+b