Of course, bookmakers notice players who bet on opposite outcomes of the same event. Especially when betting in real time, when the event has already started. There is nothing forbidden about it, so players get their profits. But when playing live, it is not so easy to "catch a surebet". It is not very difficult to bet on one player or team with odds higher than 2, but it is difficult to wait for the opponent to win back and begin to seriously claim victory. Only in this case, the coefficient will be higher than 2.
Therefore, the bread of the "fork" is difficult, it can be called a kind of luck.
Other bookmakers fight with arbers by returning any bet in pre-match with odds of 1. This is spelled out in the rules, and the bookmaker is not obliged to explain the reason. Naturally, each such application of the rule is a blow to the reputation of the bookmaker, so the return of the bet is rarely practiced. But even the presence of this rule scares away "arbers".

If the player constantly wins, then his account is checked by the bookmaker's security service. In the event that the analysis of the account does not reveal anything suspicious, then the player is allowed to continue betting. If deliberate earnings on surebets are detected, the player's account can be blocked forever, but with a refund. This is a kind of denial of service, which is also spelled out in the rules of almost every bookmaker's office. The bookmaker is also not obliged to explain the reasons for the refusal of service.