MySQL - delete rows with a duplicate field

how can i execute a (one-time) SQL stmt that deletes rows that have a duplicate value...
something like:

DELETE FROM T WHERE A=another(A)

thus
ID     A
1      x
2      y
3      z
4      z
5      y

would delete either row 3 or 4 (doesn't matter to me which, just not both :)    )

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