FUNCTION DuplicateKey(tcTag, tcTable, txPkExpression)
	* Check for duplicate, according to a certain index tag.

	* Parameters:
	* tcTag: name of index tag.
	* tcTable (optional): name of table. Required if working with views.
	* txPkExpression (optional): primary key expression. Required when working with views.
	*   This allows to omit current record from checking of duplicate.
	*   Can't be used to check uniqueness of primary key (but I use non-business-value keys,
	*   automatically generated, anyway).

	* Returns: true if another record exists with the same value(s)
	* (according to the specified index tag).

	* Additional comments:

	* By intercepting for this duplicate value before Visual FoxPro does (through Primary or
	* Candidate Indices), the user can messages that are clearer than "Trigger failed"
	* (at least with Visual Extend, "Trigger failed" doesn't give more details).

	* The function can also be used to check duplicates in FoxPro 2.x;
	* just change "local" to "private".

	* Of course, depending on the index, this function may check whether the combination of
	* two or more fields is unique - however, only the index tag is passed as a parameter.

	local lnSelect, lcDbf, lnRecno, txEvalExpr, llDuplicate, lcExpression, lcOrder, llWorkOnView
	llWorkOnView = not empty(tcTable)
	lnSelect = select()
	lcDbf = iif(empty(tcTable), dbf(), tcTable)
	lnRecno = recno()
	lcOrder = order()
	if not llWorkOnView
		set order to (tcTag)
		lcExpression = key()
	else
		select 0
		use (lcDbf) order (tcTag) again
		lcExpression = key()
		use
		select (lnSelect)
	endif
	txEvalExpr = eval(lcExpression)
	select 0
	use (lcDbf) order (tcTag) again
	seek txEvalExpr
	if found() and (llWorkOnView and eval(tag(GetPkNum())) = txPkExpression;
			or not llWorkOnView and recno() = lnRecno)
		skip
	endif
	llDuplicate = (eval(lcExpression) = txEvalExpr)
	use
	select (lnSelect)
	set order to (lcOrder)
	return llDuplicate
ENDFUNC && DuplicateKey