Hi there,
In the OOB Editor every single unit is unique : it cannot be reused elsewhere in the same OOB except when the mention "can be duplicated" is added in the text box under the image of the unit => it's the case with Austrian Grenadiers (Grenadieren Division) for example.
The solution is to use a second unit (of the same quality), to rename it and to change its colors to make it similar to the first unit (this late feature can be done in the panel dedicated to the unit by clicking on the "Distinctive colors" link).