Idea: HashCodeCombiner... extend RuleExpression

public int RuleSetId { get; set; }
public override int GetHashCode()
{
	var combiner = HashCodeCombiner
		.Start()
		.Add(RuleSetId)
		.Add(Id)
		.Add(Operator)
		.Add(RawValue);

	return combiner.CombinedHash;
}

and extend RuleExpressionGroup

public override int GetHashCode()
{
	var combiner = HashCodeCombiner
		.Start()
		.Add(Expressions.Select(x => x.GetHashCode()));

	return combiner.CombinedHash;
}

Add "expression.RuleSetId = entity.RuleSetId" to RuleProviderBase.ConvertRule and group.GetHashCode() to CartRuleContext and use it instead of expression.Id when creating the key.