if i have the following columa

aa 111
bb 222
aa
cc 333
dd 444
aa
aa 321
cc
cc

from above if it huag data and i want ensuar that all aa have same key
account whuch 111 and other key account and if aa has more than one key
account it show wrong or any thing like B7 it has wrong key which 321
how i cank make it by excel