Double clicking on a column head expands the corresponding column width in order to get all the texts in that field to be entirely visible.

That's not wrong, but in case that one txn in that register has a very long field, it can happen that the expansion produces that the corresponding column gets very wide, and that the rightmost columns get hidden.

If the user didn't want that abnormal expansion, he/she has to ripristine the column width manually, which is an annoying operation.

If the user doble clicks again, nothing happens, because the double click has only that effect.

On the contrary, I think that on the second double click the column widht should go back to the preceeding value. The double click would become a toggle command on the column width.

Alternatively, doble clicking on a column head in the register could better produce sorting by that column. That would be a more standard behaviour for double clicking.