If B4 contains the same number as A3, how can I put the number in A4?

Here is my spreadsheet:

A B
1 0082766
2 0082767
3 0082768 0082768
4 0082768
5 0082768
6 0082768
7 0082768
8 0104361
9 0104365
10 0104367
11 0104368
12 0104370
13 0104409 0104409
14 0104409


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This is the result I'm looking for:


A B
1 0082766
2 0082767
3 0082768 0082768
4 0082768 0082768
5 0082768 0082768
6 0082768 0082768
7 0082768 0082768
8 0104361
9 0104365
10 0104367
11 0104368
12 0104370
13 0104409 0104409
14 0104409 0104409