1v1's are allowed as long as both parties agree to it. Meaning Player X asks Player Y if they want to 1v1 and Player Y says sure, on go Player Y dies and Player X gets warned for RDM? that's not how it works just letting you all know.
If players are having some banter (Killing each other in a base and defibbing) or else ware as long as it isn't major or affecting/causing any issues on the server its fine just let them be but if it gets out of control then do something. I kill my friends with banter all the time, heck I shoot EMP in the face in spawn for no reason because he looked at me. Banter goes throughout the server hence why it is called Semi-Serious Role Play not everything is supposed to be taken seriously. Be more lenient is all I say and ask me before you TP and I wont shoot you