Geordie613: If the rationale for not wearing tefillin is because Chol HaMoed is an Ois [working with the Nigleh world, not the Kabbalah], then Chol HaMoed would be equivalent to Shabbos and Yom Tov, and the Gemara Eruvin 95 is mefurash that one who wears tefillin on Shabbos lsheim mitzvah transgresses baal tosif.
Performing any mitzvah outside its time frame is ba’al Tosif.
In your example of Shema, one who says Shema twice during the zman isn’t oiver because repeating a mitzvah in its time is permitted (like shaking an esrog a second time) according to most rishonim. Saying Shema outside the zman with intent to be yotzei would constitute baal tosif. (R’ Shloime Bloch was מעיד that the Chofetz Chaim told him that saying Shema outside its zman is assur because of baal tosif. The Chazon Ish and Reb Moshe both disagreed, but only because Shema is always considered zmano of Limud HaTorah – כאילו קורא בתורה. I don’t really understand their sevara; bli neder I’ll post a link.)