Python 2, 105 bytes
s="ABCDEFGHIJKLMNOPQRSTUVWXYZ"*2
for i in range(51):print s[:i]+s[i::-1]if 26>i else s[:-i-1]+s[-i-3::-1]
Thanks to @LeakyNun and @manatwork for pointing out a few byte saves.
ospahiu
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