A (Mensa!) Puzzle Book by Walter Mackay called Repeat-Letter Sudoku caught my eye this week.
This book offers puzzles where there is one (or more) doubled letter(s). In the Cartoon, there are two repeated letters: U and A. No matter. Just do the Sudoku Puzzle as usual and treat each of the pair as two different numbers.
It may help to write the same letters in unique ways: one in block letters and one in cursive or lower case. Alternatively, you can just keep count in every row, column, and block that duplicated letters show up exactly twice.
I like the effect of the no duplicates rule in the original sudoku being relaxed. It forces me to look anew at the puzzle and how it can be done, without prior built up procedural blinders. Interestingly enough, this particular variant appears studiously ignored by the rest of the internet.
I suppose if this carried far enough, it becomes simple to solve (e.g. All but one letter is duplicated 8 times). Don’t do too many of these; you may have to complain to your Eye Doctor that you’re seeing double!