When I place my hand palm down and lift my index finger up, I notice the tendon splits into two parts. I can see both parts moving. No one else seems to be able to do this. Do I have two of the same tendon?

It's possible, but more likely you have something called a tendon slip. There is a main tendon and some separate fibers alongside the main tendon. They are usually inside
the same tendon lining or sheath. Since you can see them move separately, they may each have a separate covering.

From studies of anatomy we know about three percent of the population has some
differences from the norm. Sometimes this is a missing tendon or muscle. Sometimes it's a fused bone or missing bone. Small variations in placement of tendons is very possible.

Reference:

Yukio Abe, MD, PhD, et al. Extensor Pollicis Longus Tenosynovitis Mimicking de Quervain's Disease Because of Its Course Through the First Extensor Compartment: A Report of 2 Cases. In The Journal of Hand Surgery. March 2004. Vol. 29A. No. 2. Pp. 225-229.

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When I place my hand palm down and lift my index finger up, I notice the tendon splits into two parts. I can see both parts moving. No one else seems to be able to do this. Do I have two of the same tendon?