The Leader and Mech are the same, but I can't add it to a Squad.

The following are possible reason why you cannot add the Mech.

1. The selected Mech is mining
2. They are the same Mech, but different types
3. They are the same Mech and Type, but it is assigned to a different Squad

Mechs that are currently mining can be assigned to a Squad after mining is complete.
Also, Mechs may be the same, but if their types are different, they cannot form a Squad.
(Stalker Type A and Stalker Type B cannot be in the same Squad)