[Borek]

If add now again 100 ml 0,05 m  H3PO4 again the volume increase to 200 ml

Molarity changed to 0,075 M

Now do the math adding 0.1 M H₃PO₄.

[Hunter2]

pH drops more

pH = 2,1 + log (0,025/0,075) = 1,62

Molarity = 0,1 M

And now, what should it proof, pH is changed more and molarity is in this case the same but ratio phosphate and phosphoric acid is different.

The only thing what we can say it is possible to have a constant molarity with different pH by choosing different ratios of phosphate and phosphoric acid.
But it doesn't say if I add some phosphoric acid to a phosphate solution the molarity will be the same, only if molarity of both compounds are the same.

At least M total = M(H3PO4) - M(H2PO4-) + (MH2PO4-) - M(H3PO4)

0,1 = 0,05 + 0,05
0,1 = 0,025 + 0,075
0,1 = 0,01 + 0,09
etc.
pH will result in the quote in HH equation.

[Borek]

0,1 = 0,05 + 0,05
0,1 = 0,025 + 0,075
0,1 = 0,01 + 0,09

And that's the point - these are all 0.1M phosphate buffers.

Question from the very beginning was whether it is possible to keep the buffer concentration constant while adding acid. OP doubted, but now that you did the math you have proven that it is perfectly possible.

So the addition of phosphoric acid should affect the final concentration of phosphate buffer. right?

The correct answer is: it can, but it doesn't have to. What happens to the buffer concentration when adding acid depends on the acid concentration. It is not a simple yes/no question without additional clarification.

Actually preparing buffers of a constant concentration by mixing volumes of acid and base of the same concentration as the required one is a standard lab procedure. Makes calculations easier and removes source of possible errors, as the buffer concentration can't differ from the expected one. You still have to check the pH to be sure, but that's the easy part.