Thanks. Doesn't sound like a desirable trait.
Nah.
It just means that Ruger accidentally reused a serial number that had previously been used on another firearm. To keep their records as straight as possible (or as legal as possible, depending upon the year)*, they added a 'D' to your serial number, to make it unique.
Ruger pretty much
NEVER creates duplicate serial number replacement frames. They just ship you a new version of the product, with a new serial number.
As mentioned already... Given the right crowd, a D prefix can bring extra $$$$.
But, if you want some serious cash, track down the original 'mate' with the un-prefixed serial number, and sell them as a pair. (Good luck. There are only 133 million households and 129,800 gun dealers to check, if it even still exists.
)
*(Ruger did 'accidentally' ship some Single Sixes, and possibly other models, when they accidentally over-ran previously-used blocks of serial numbers in the '60s. Similar situations, without shipping them, occurred over the following decades. There are Single Sixes, Super Single Sixes, Bearcats, Blackhawks, Mk Is, and 10/22s that have been verified to be all-original [even NIB/unfired] that have D prefix serial numbers.)