What is the cause of some Mozart works bearing a double KV. number?

[Enrique]

For example, for the Sinfonia concertante, we have K.364/K.320d. By examining Wikipedia's article "Köchel catalogue", I see the first number, 364, is Köchel's original designation, whereas 320d is its designation in the 6th edition of the cataloge. I must confess it's the first time I have noticed such a thing. So, could I say that all of Mozart's works are currently being identified by a double number, the first corresponding to the original KV and the second to its sixth edition, save in cases where those numbers match? And how old is this (for me) new way of indexing his works?

ADDENDUM: I express my wishes that Roehre be coming back soon, wishes already having been made explicit by some other fellow BRSers.

[Chris]

Yes, you have that exactly right. How long has it been that way? Since 1964, I imagine, when the 6th edition was published.