The decision rhodesia_charter awards Rhodesian and Malawi provinces to the UK even when other countries own them. The code that does this is here, which does not check if the provinces are already owned by another tag:

The following pair of screenshots were taken on the same game day. Before the decision:

After the decision:

Potentially, this decision should check if the provinces are empty or not, and if they are part of a colonial race, although truly I do not exactly see why this decision should secede_province at all. The UK will normally outrace all other tags at obtaining these provinces, but if it cannot, then the game should reflect that. For this reason, I suggest that the best solution is to simply remove all of the secede_province statements.