Jojo, sorry but I don't know your level of experience/knowledge of the various arts, so if I seem a bit patronising I apologise.

Aikido is largely based upon the art of Daito Ryu Aikijutsu. The founder of aikido (often reffered to as O'Sensei) was taught Daito ryu by Sokaku Takeda sensei, who revived the art in the late 1800's.

O'sensei changed his art as he aged, to suit his changing ideals, and there are now a number of styles within the art. Some of the older styles (Aiki budo, Yoshinkan) are still quite closely related to aikijutsu, which has many similar techniques. Aikijutsu could be said to be the father art to aikido.

The Korean art of Hapkido also has strong links with aikido, and employs much of the same nage waza. It also uses a lot more strikes and kicks than aikido, being similar in many respects to TKD. This art is probably the closest to aikido of all the "modern" styles.

So, yes - aikido is unique, but only in the same sense that karate is unique, or ju jutsu is unique. There are still a number of variations within the styles, and each one oi those could also be said to be unique, despite sharing a lot of common ground.

Hope that's of some help

Budo