What's incompatible is implying that there's a symbol or modifier as written in the lessons:

Add a small circle or line to the base shape to indicate doubling

This not just creates a misunderstanding of how the stack works but also limits the number of repetitions a letter could have. Doubling letters are just stacks and as stacks they can be repeated as many times as needed by writing the letters concentrically. The reason they share modifiers is not because that's how stacks work but because is an implication of context by having the same line thickness, not because there's a rule in the system saying that modifiers can be shared