Another method to accumulate decorations is if you play regular game, take all your paintings and shields to your inventory and then when you start a NG+ you will automatically have them ready to when you will reach Corvo Bianco again (meanwhile just store them in your stash).
And yes, while some of them you will get duplicates in NG+ (if you choose), you could get an alternate design for Geralt shield and his portrait.
And yes, while some of them you will get duplicates in NG+ (if you choose), you could get an alternate design for Geralt shield and his portrait.