What is the function this united Ds instead of a D

Note normally used to seeing them connects as :



Without a total schematic not easy to answer your question.

If connected to a I/O pin of a cmos part its normally to insure pin
never violates Vdd >= Vpin >= Vss. To insure no significant current
flows thru onchip parasitic diodes which can cause CMOS latchup.



Regards, Dana.

Is this symbol connected with a real part?
Not sure what you mean by a "FET transistor". Did you just make that term up? Literally it means "Field Effect Transistor Transistor".

Simple components like diodes are sometimes made with more than one component in a package. It's saves a bit of cost, board space and assembly time over having two separate diodes.

Here is an example:- https://www.diodes.com/assets/Datasheets/ds30234.pdf

There are versions that are common anode, common cathode and series.