Is it a duplicate Date?

The year 1879 has two rows with no details. Even the description is the same in both (type C Bust, Type IV Reverse). Only the frequency is different.

Is it an error or am I missing something?

One was meant to be Proof and the other meant to be Prooflike Restrike.  Looks like about 11 members on each line. 

rsirian1

One was meant to be Proof and the other meant to be Prooflike Restrike.  Looks like about 11 members on each line. 

 

The date list from which you have matched has (B) for those two years but actual coins from that year is present without 'B' which is matching with this site. So, is it a variety?

Just think it's a plain old error by the person who added those two years, not adding the mint marks and not adding the correct descriptions. The referee should fix.

Yes, as you can see on rsirian1's image of SCWC all the mintmarks are between parenthesis (except for the C mintmark) which means there are no B's on the coins so the listings on our catalog should indeed be fixed.