Duplicating Values in a column

Hi all,

I need to duplicate values in a column, and keep them in the order they are. For example in Column A I have:

3
6
2
4
12
45
13

I want to duplicate them, so in Column B for example it would look like:

3
3
6
6
2
2
4
4
12
12
45
45
13
13

Seems like a pretty simple need but I can't figure out the formula.

Any idea?
Thanks