Doug,
Running 4.6.6 with Windows 7 x64 Ultimate.
I find that the caption is retaining the previously loaded one as well as the new one.
Example - new caption is 'Steam Train' and the old one (not showing in the stationary pad) is 'Stavros' - caption that appears on my image is 'StavrosSteam Train'.
Is this a bug or am I doing something wrong.
To view the caption for a particular image, you need to use the IPTC Info dialog, not the IPTC Stationery Pad. If you use the IPTC Stationery Pad multiple times and you have the 'append' checkbox set (it's a checkbox with a '+' on it) and you edit the caption, and then apply the IPTC Stationery Pad, you'll be appending new data onto any existing captions.
Perhaps that's what happened to you?
Open up the IPTC Info for your image (click on the 'i' button while your mouse is hovering over your image) and what does it say? If it isn't correct, fix it and OK the dialog. Then open it up again. Is it correct now?
HTH,
-Kirk