Below, you can find a walkthrough of the "Spider-Men" side quest in Marvel's Spider-Man.
Mission's starting location: Harlem
Reward: 1500 XP
After you talk to the man, he will thank you for saving his daughter, which will be weird since Peter didn't actually save anyone. This has to be investigated - go to the location of the next crime. Talk to the policemen, examine the beaten criminals and talk to the woman.
Time to go after your doppelganger. Go to the marker and talk to the guards. Scan the environment. Approach the dropped fire extinguisher and analyze the substance. The solution is shown above.
Follow the highlighted substance. Eventually, you will notice another scene of a prevented crime. Interrogate the suspect and go to the roof. Take a look at the beaten people and the drugs left there. You will be able to notice the tracks of the false Spider-Man. Enter the fight with Fisk's goons - together, you will defeat the gangsters. The police will arrive shortly. Mission successful!