Can Akshan take control of an equipment that is already equipped on an enemy?

Yes, Akshan can take control of an equipment that is already attached to an enemy unit.

According to the RiftJudge FAQ #1513 and FAQ #657, Akshan's ability allows you to target an equipment even if it is currently attached to another unit. When the ability resolves:

1. The equipment detaches from its current unit.
2. You gain control of the equipment.
3. The equipment moves to Akshan and becomes attached to him.

As noted in the FAQs, while you control the equipment (as long as Akshan remains on the board), you can use it as if you played it, but your opponent cannot use its abilities or pay to equip it back to their units. Once Akshan leaves play, control of the equipment returns to its original owner.