And your point or question is?

Ok a stab, why/how did this happen? One thing you need to be aware of is
that it is possible to get the camera to change the sequence by putting the
card back in with a different image number on it. The camera checks the
card for the highest number on it and can rest it's numbering starting off
from than number. An issue to be aware of if you routinely don't clean
cards or if you swap cards between cameras with cleaning them out.

For instance, delete all the files on the card then transfer #500.jpg back
and you could find the next number in sequence will be reset to 501.
--
Ed Ruf (Usenet2@EdwardG.Ruf.com)

I don't know why you would have dup numbers on the same card, but if
you use the camera to delete pictures as you are taking them, it
sometimes makes the numbering go funny, especially if you take some
pictures, sit down and delete what you don't want, then take more pix,
etc.

According to Joan <Joan@home.tosh>:

    Do you mean that when you downloaded *some* shots, you deleted
those from the media and left others which might have had higher and
lower image numbers?

    And did you do this with the card in the camera, or by moving to
to a card reader and then moving it back.

    Note that I use the D70, not the D50, and thus use CF cards, not
SD cards, but otherwise things should be about the same.

    O.K.  There are not enough images in that first folder to
account for the creation of a second folder.  Normally, that happens
either when there are 999 images in a folder, or when the image number
reaches 9999.

    What I *think* happened here is (assuming that you deleted some
images in a card reader on the computer and then re-inserted the card in
the camera) that the camera started reading through the highest-numbered
folder looking for the end after finding the first image number in that
folder.  What happened is that it reached (say 5250) and saw that 5251
was not present, it set the number to 5251, without bothering to see
whether there was anything beyond that (after all, that takes longer to
get the camera started up if it does that).  Then you took 5251, and the
next shot (5252) it discovered was already present in that folder, so it
created a new folder and put the remaining images from that session
there.  This prevents overwriting of existing image files, though it
does confuse the image-numbering sequence somewhat.

    I've not encountered this, yet, with my D70, in part because
I've been using 1GB CF cards, and an image size which will not get as
many as 999 images on the whole card, let alone in a single folder.
However, I've added a 4GB CF card to my collection, and depending on the
image size, I can get well over 1000 images to a single CF card.  (I had
been shooting with Medium/Fine settings, and going to RAW only for
special cases.  On the 4GB card, that will get over 2K images.
Large/Fine gets over 1.1K, and only full-time RAW will get be below 1000
images about 716).

    I used to have a script (unix system) which downloaded the
contents of the DCIM/100NCD70 folder (yes, I know that on Windows, it
would be seen as DCIM\100NCD70).

    Now, since that risks not downloading everything from the card,
my script uses tar to copy the entire tree of directories and their
contents into the unix system, so I don't have to worry about any
accidental duplication of filenames resulting in something lost.

    And, as soon as the download (and duplication to a second hard
disk) is complete and verified, I put the CF card back into the camera
and format it to start over.

    I also reset the image number back to 0001 at the beginning of
each year.

    If the image number goes past 9999 in a shooting session, it
will (of course) create a new folder to make sure that things do not get
overwritten.  If I need to combine them back to maintain a proper
sequence later, I can rename the later ones from DSC_???? to DSC1????
and on as needed.  

    I presume that you first safely copied the duplicated filenames
to separate directories -- or renamed them first?  I have this image of
the files being lost when you reformatted the card.

    I do sometimes delete obviously bad images from the card, if
there is a question of room, but I tend to keep the card in the camera
until I am out of room or have another reason to change CF cards.  I
suspect that your problem occurred when you re-inserted the card after
deleting some files via an external computer. (It may be that the camera
keeps some information on the card to keep it from duplicating numbers,
and this was corrupted by the computer downloading the files.)  I've got
things set up so my cards mount read-only (again, a unix system), so I
*can't* corrupt the filesystem -- and this forces me to always use the
camera itself to format the cards.  (Besides, on a unix system, I have
too many options for how to format a card -- not just the FAT
filesystems which the cameras use. :-)

    That should not be the cause of the problem -- but if you did
delete images from the CF card in an external computer, and then
re-install it in the camera, *that* might have triggered the problem as
I described above.

    As I mentioned above, I *do* delete images from the CF card
before I am done with it -- but I do so using the camera, and I've never
had the camera loop back to re-use some deleted numbers.

    I hope that this helps,
        DoN.

Simple fix
Set the camera to consecutive file numbering instead of having it start
from scratch each time you format the card. This also helps you ID how
many clicks the shutter has done so you can replace the camera before
reaching the shutter life expectancy... 60,000 for most consumer DSLRs.
The rate some people around here take pictures that'll equate to end of
life in about 3000 AD. Except for Anika1980 - AKA Bret Douglas. He
reaches that count every second lunch time.!