Your design is overparameterised, which is explained in my paper here: https://www.frontiersin.org/articles/10.3389/fnins.2016.00270/full
It’s not an issue, but you have to be very careful with your contrasts to be sure they’re testing what you think they are. Also, if you’re trying to specify a repeated measurement design make sure you read my paper on the topic to be sure you’re performing inference correctly: https://www.frontiersin.org/articles/10.3389/fnins.2019.00352/full
Best wishes
Martyn
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