Since because means for the reason that, the construction the reason … is because is a redundancy. In most cases, you can eliminate the redundancy by changing because to that.
| Incorrect: |
The reason for the shortage is because our textile mills can’t expand overnight.
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| Correct: |
The reason for the shortage is that our textile mills can’t expand overnight.
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Sometimes it is best to rephrase the sentence.
| Incorrect: |
The reason they think this way is because they do not understand marketing.
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| Correct: |
They think this way because they do not understand marketing.
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