Hi BansalT.
Generally, when you reset an official practice test and take it a second time, you see some repeated questions. As a result, people's scores on retaken official practice tests are often inflated, and the experiences they have on the tests aren't quite representative of the GMAT experience because the stress is lower.
However, the first two official practice tests have a large question bank. So, in retaking those two, you won't see many repeated questions, at least on the first retake. Thus, retaking the first two practice tests is worthwhile.
Even retaking the other four can be somewhat worthwhile, particularly if you got a very low score the first time and thus will see new questions if you're scoring higher. However, retakes of the other four aren't great.
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