DbChecker: add ability to flag duplicate PVs as intentional

This ticket originates from ISISComputingGroup/IBEX#1760

The Lakeshore336 has a few PVs whose definition is spread over two files (see #1760 for explanation of why), and this is intentional.

It may be good to have a way of flagging PVs as intentionally duplicate, so the dbChecker doesn't warn about them.
While doing this, please apply this flag to the duplicate PVs in the lakeshore336.

ALTERNATIVE would be to refactor the lakeshore336 so there is no duplication. Technically the duplication comes from the vendor branch, where the driver uses all the features of the device. But as we're only using some features we could refactor to remove the duplication.
After discussing with Matt we concluded it's not worth it (maybe we'll need the extra functionality in the future?)