Need help with cat...new here

[Peanut&Charlie]

Hello,

I have 2 male cats that are 4 years old, Peanut and Charlie (hence the name). We have had them for 2 years now. This summer there has been an increase of stray cats in the neighbourhood and in our backyard. We noticed our cats spraying plants around our yard when we took them outside. This spraying was then found inside. On our stove and on our bed. We have only caught the one cat spraying inside. The thing is after we clean Peanut must still smell from when he previously sprayed and will spray again. Is this normal? I thought they would recognize their own smell? It has been an expensive summer to say the least. We have tried so many different things and I think the next step is a trip to the vet. I did read up on the difference between just not using the litter box and spray but he still uses the litter box.

As well, last summer we had no issues with spraying inside or outside. We took these 2 cats in from my sister. We are very much attached and I could not get rid of them but would like to fix this situation.

Thank you in advance!