[FIX]Fixed the issue of duplicated one2many lines, weired behavior when you add one2many line and then add another line withour filling required field and then click on save, then remove added one2many line for which we didn't filled required data and then save it, you will get duplicated lines.
[MERGE] [FIX] FormView: avoid multiple creations of the same line in o2m fields when the save action fails (e.g. forgot one required field)

This is done by removing the added action 'prepend_on_create' that can be present as many time as we pressed the save button