I suggest we hold a Clan War.

it's not written in the rules like this

Each player from one clan plays against each player from another clan twice.

I don't understand what the last words "twice" mean - twice in a row or doubled.

But "playing against each player from another clan" definitely means that you shouldn't play against members of your own clan. :) otherwise, it wouldn't be a clan war :))

I haven't participated in clan wars, but I was told that's how it works.

Clan war is a competition between clans, meaning a game between people belonging to different clans.

However, in offline mode, everyone can play the same tournament map, but then the overall scores of players from one clan are compared with the overall scores of players from another clan. That's approximately how a clan war works.

Although, of course, everything depends on the organizer of the tournament.