I've also done rules where you only collect money for your territories at the start of your turn, instead of the end. This prevents people from collecting twice for territories in MOST cases.
The out of box rules are a little un-realistic, since you collect twice for territories that see much combat. In real war that territories produced very little because the assets was bombed and destroyed and on east front it was also used the scorched eart taktics, wich is you blow up or burn everything.
The most realistic rule would be the one from A&A D-day, where all players can share territories. Like germany attack Ukraine with 4 inf, and one of the russian inf survive after first round of combat. Now germany choose to not attack again, and so Ukraine is divided between german forces and russian forces, and make no income.
The best rule is of course to collect income before combat move, but then no players would have cash that could be SBR'ed. You would need to wright an essay on a pad about who lost how much during SBR. So a better rule is as mentined above, no income from territories that are contestet or recently invaded.