Learn English – Is ” Are there the person who have a doller?” grammatically incorrect

The first sentence is correct as although "father's grandfather" and "grandfather's father" are referring to the same person but at the time of speaking/referring, they are the instances of a singleton personality.

Further, the grandfather's father instance is older than father's grandfather( considering the natural timeline). Thus, due to different instances of the same person for two different perspectives, "is" can't be used in this context.

Strictly speaking, there is no problem with the grammar of that sentence. It fails on semantic grounds.

I'm looking at a man standing there.
I see a man standing there.

The verb to look expresses an action. We call these dynamic verbs. The verb to see (when used as a verb of perception) expresses a situation. We call these stative verbs. This semantic distinction has nothing to do with grammar and very little to do with a word's definition.

Many native speakers rarely use a present continuous construction with a stative verb.

As an expression of perception, to see is generally stative. As an expression of romantic involvement, to see is dynamic. We tend to interpret "I see someone" in the perceptive sense, but "I'm seeing someone" in the romantic sense.