Python 2.7, 136 bytes
x=[chr(y) for y in range(65,91)]
for y in range(25):print ''.join(x[:y]+x[y::-1])
for y in range(25,-1,-1):print ''.join(x[:y]+x[y::-1])
Python 2.7, 136 bytes
x=[chr(y) for y in range(65,91)]
for y in range(25):print ''.join(x[:y]+x[y::-1])
for y in range(25,-1,-1):print ''.join(x[:y]+x[y::-1])