أَخَذَ, (Ṣ, A, L, &c.,) in the first pers. of which, أَخَذْتُ, [and the like,] the ذ is generally changed into ت, and incorporated into the [augmentative] ت, [but in pronunciation only, for one writes أَخَذتُّ and the like,]
I doubt it's rare. To me, such a rule doesn't exist.
Your explanation would be correct if the last letter of أخذَ would have "chaddah" such as أخذَّ but it doesn't. "Chaddah" indicates two letters which the first one would be still (have a "soukoun") and the other would be formatted "moutaharek". So if the verb was to end with "chaddah"- its extracted form should be أخذذَ and that's when the "rule" would apply.
For pronunciation, compare اتَّخَذتُ with أَخَذتُ. Notice that the first form of the verb has "chaddah" while the second hasn't.
A clearer example of the last matter is فَعَّلَ and فَعَلَ. Also, شَغَّلَ (make a person/persons work forcefully) and شَغَلَ (took a certain position in the society (in a job for example)).