Kornelia Ender:
http://histclo.com/imagef/date/2008/05/ender01s.jpg
They certainly swept up the olympic medals over some twenty years (late sixties to late eighties) - at Montreal 1976, 10 out of the 12 individual gold medals, for example.
Long suspected by many swimming coaches but only confirmed on the discovery of the DDR's athlete training records after the demise of the USSR.