Optionally drop duplicate fnames from layout

[apetro]

In MyUW, duplicate (same fname) portlets in a user layout is an error every time. This situation currently can arise e.g. when adding an item to the default layout that the user had already added to their layout.

In MyUW, it would be advantageous to simply suppress subsequent instances of an fname in a user layout. Don't show them. Out of sight, out of mind, no worries.

Conceivably other uPortal-home adopters might value being able to have multiple (differently preferenced, presumably) instances of a given fname in a user layout, so maybe this de-duplication feature if added should be turned on and off with a feature flag.