I'm sorry, but I cannot think of any word to define the mistake with 'alloweded'-- it is just a slip of the tongue, it seems to me. No exercise is needed; just explain to the student what has happened (the student has duplicated the -ed ending in error). The pronunciation of /th/ as /s/ is a common one; it happens with many learners who do not have the /th/ sound in their native language.
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