Cloning synchronises each target date with the source rather than just appending:

• Meals on the target that are not in the source and have no orders are removed.
• Meals on the target that already have orders are always kept, even if they are not in the source.
• Source meals missing from the target are added (inactive recipes are skipped).
• Meals present on both stay untouched.

So after cloning, a target date equals the source menu plus any already-ordered meals. Note that the dialog text says days with an existing menu "will not be affected" - in the current version the behaviour described above is what actually happens, so review dates that were hand-built before cloning over them.