I would think this is your Duncan:
Duncan Keir
Born: 6 Apr 1822, Christened: 28 Apr 1822
Recorded in: Drymen, Stirlingshire, Scotland
To Duncan Kier and Janet Stewart
I couldn't find any Duncans born c1771, the following was the nearest:
Duncan Keir
No date of birth listed, Christened 25 Mar 1768
Recorded in: Balfron, Stirlingshire, Scotland
To John KEIR and Agnes GRAY
This Duncan was one of six children born to this couple and all born in Balfron. His siblings: (no dates of birth listed for any of them, only christenings)
Daniel, christened 4th August 1758
Christian, christened October 1763
Margaret, christened 15th January 1766
Agnes, christened 15th April 1770
George, christened 7th January 1774
Sometimes families followed naming patterns for the children - were any of the above listed names carried on?
It's obviously not a definate way of identifying different families but it can make a huge difference when you see the same names being used repeatedly down through generations.
I've sent you a PM with some more info.
Anne