The wifi internet router is connected directly to Internet cable. The main Pc is attached to it via LAN cable, while 1 laptop and 3 mobile phones connect to it via wifi.

Whenever 2 or more devices connect via wifi, the internet connection breaks after one minute and internet connection resets.

I tracked this behaviour for weeks, and came to conclusion: It seems like some 'device 1' got IP then it went to suspend mode. Then 'device 2' connected to router and got the same IP. Then the 'device 1' woke up from suspend mode and tried to use his old IP. The router sees that 2 same IP addresses exists and automatically reset internet connection.

Is this possible? Have I tracked the problem correctly and how to solve it?

The router is set to lease 100 IP addresses to devices who try to connect. The password is strong and no hacker's device is being connected to my wifi network. Tried changing password and AP's name.