@Pistol7Pi: your script based on some secret knowledge (unknown to me)

set var1 {YNC 1 2;K1_SP4_SPA_CLEANUP_DIAMETER UGT0101_008;K1_SP4_SPA_FACING UGT0101_008;SYNC 2 2;K1_SP4_SPA_ROUGHING UGT0101_008}
set var2 {SYNC 1 2;K2_SP4_CHECK_DIAMETER UGT010_0232;SYNC 2 2}

# some secret logic (at the moment returns 0 first 2 times called and always 1 hereafter):
set i 0
proc secret_knowledge {} {upvar i i; expr {[incr i] > 2}}

set var1 [split $var1 ";"]
set var2 [split $var2 ";"]
set res {}
foreach v $var2 {
  if {[regexp {^SYNC } $v] && [secret_knowledge]} {
    lappend res NOOP
  }
  lappend res $v
  if {[regexp {^SYNC } $v] && [secret_knowledge]} {
    lappend res NOOP
  }
}
puts "expect:\tSYNC 1 2;K2_SP4_CHECK_DIAMETER UGT010_0232;NOOP;SYNC 2 2;NOOP"
puts "result:\t[join $res ";"]"