import re
arr = ['prefix a b c 123 c 123', 'prefix a b c d e 123 d e 123']
rx = re.compile(r'^prefix\s+(.*?)(.+?)\2+$')
for el in arr:
r = re.sub(rx, r'\1', el)
print (r)
aW1wb3J0IHJlCgphcnIgPSBbJ3ByZWZpeCBhIGIgYyAxMjMgYyAxMjMnLCAncHJlZml4IGEgYiBjIGQgZSAxMjMgZCBlIDEyMyddCgpyeCA9IHJlLmNvbXBpbGUocidecHJlZml4XHMrKC4qPykoLis/KVwyKyQnKQoKZm9yIGVsIGluIGFycjoKCXIgPSByZS5zdWIocngsIHInXDEnLCBlbCkKCXByaW50IChyKQoJCg==