In a forum that is not sudoku-related I found a nice variant called Tricky Sudoku. It is a subvariant of Miyuki Misawas Pair Pair Sudoku.

Her a puzzle I made of this variant:

In each row and column every digit occurs only one time, in the 3x3 boxes there are 3 numbers once , 3 numbers twice and 3 numbers doesn't occur.

Uwe