@User your decoder3x8 has 2 instances with same name, one is called X1, the other is called 'x1' (lowercase) since spice is case insensitive they appear as duplicate. Xschem does not report this as an error (although i can improve that) because upper / lowercase is allowed in verilog designs. Rename the 'x1' as something different (X9) do the same for 'x2' and 'x3'.