There are lots of posts on here about DPC levels.
Years after the mobility standards Part M were introduced in the Building Regulations (the cause of this defect) housebuilders are still not ensuring the dpc is at least 150mm (2 brick courses) above finished ground level.
Unfortunately, the Building Regulations do state "should be" and not "must be". The NHBC standards are similar.
Worse, unless the home is affected by this (damp on walls inside and/or out) housebuilders can and do, dodge doing anything about it.
Dpc "one on top of another in the same mortar joint" serves no purpose at all.
I expect they could be talking about a cavity tray which would help stop damp above ground level on the inside walls. They may also be talking about another separate dpc above the one level with the ground which is OK.
But with what you have said it is more likely they don't know what they are doing and are now trying to pull the wool over your eyes.
Have the home professionally inspected and snagged.
Who is this housebuilder?