Wisdom/experience/advice required please!

So the following situation occurred:
- I was travelling on a flight that was delayed, and I decided to return the next day after being rebooked for the following morning (it was late, I couldn’t travel at the other end because arrival would be so late). This was due to a technical issue whereby another flight was being flown to take over the flight. The flight (on new aircraft) landed 3 hours and 1 minute late.

- Flight the following day was cancelled due to technical issue, and I arrived 2 hours 13 minutes late after rebooking on the next flight.

- I claimed EC/261 for both flights separately, and then got separate approvals, both at a value of €250.

Since then, I have been compensated €250 for one flight, and got the compo offer redacted with the following explanation:

“I note that you have been paid compensation for the delay to flight ### on ###. We are therefore unable to offer further compensation for the cancellation of flight ###. This is classed as an assistance flight that has been booked to replace the previous disrupted flight. Under EC261/2004, compensation is only payable for one flight on each leg of the journey. This means that we cannot pay compensation for both flights.”

Is this true? I can’t find the regulation that states this.

Thanks in advance!