I'm not sure what a full-fashioned double decrease is. I think I've heard this term used for decreases that are done a couple of stitches in from the edge, where they're more visible and give an ornamental effect, but I don't know if this is an accurate definition, and I don't think that reference was to a double decrease, but rather a single decrease.
If your decrease is supposed to apear like a straight line, it's probably Centered Double Decrease; "cdd" on the abbreviations explained page.
Hope that helps.
KnittingHelp Queen Bee
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