Can anybody tell me whether the 're' in 'return'and 'repeat' is a prefix?
It depends on what you allow as derivation.
Synchronically, that is in English itself, the re- in return is surely a prefix, as there is also turn in English, from which to derive it, 're-turn'. But English has no verb 'peat' to derive 're-peat', so it can't be a product of synchronic derivation.
Taking the words' etymology, on the other hand, the re- in 'repeat' is also a prefix, as this verb comes from Latin repeto, repetĕre 'to strike again', derived from a simplex peto, petĕre 'to fall, to attack'.
Both verbs came to English via Old French, and not directly from Latin. Sometimes the sound changes from Latin to Old French make the original words less recognizable, like Latin canto, cantāre 'to sing', Old French chanter 'id.', English to chant.