How to prevent pop up message twice

[rins]

Hi Team,

I have a question about a msgbox that pops up twice instead of just one time.
Note:Excel data is refreshing via ODBC connection and validating that sheet.
Kindly help me on this.

VBA code:

private sub worksheet_calculate()
dim target as range
set target=range("c1")
if not intersect(target,range("c1") is nothing then
  if (thisworkbook.sheets("sheet1").range("a3").value="") then
    msgbox "Error"
  end if
end if
end sub