🐛 Fix G-code resend race condition

As pointed out in #24972 by @silycr, but simplified.

Marlin/src/gcode/queue.cpp

@@ -469,8 +469,11 @@ void GCodeQueue::get_serial_commands() {
 
           const long gcode_N = strtol(npos + 1, nullptr, 10);
 
+          // The line number must be in the correct sequence.
           if (gcode_N != serial.last_N + 1 && !M110) {
-            // In case of error on a serial port, don't prevent other serial port from making progress
+            // A request-for-resend line was already in transit so we got two - oops!
+            if (WITHIN(gcode_N, serial.last_N - 1, serial.last_N)) continue;
+            // A corrupted line or too high, indicating a lost line
             gcode_line_error(F(STR_ERR_LINE_NO), p);
             break;
           }
@@ -480,13 +483,11 @@ void GCodeQueue::get_serial_commands() {
             uint8_t checksum = 0, count = uint8_t(apos - command);
             while (count) checksum ^= command[--count];
             if (strtol(apos + 1, nullptr, 10) != checksum) {
-              // In case of error on a serial port, don't prevent other serial port from making progress
               gcode_line_error(F(STR_ERR_CHECKSUM_MISMATCH), p);
               break;
             }
           }
           else {
-            // In case of error on a serial port, don't prevent other serial port from making progress
             gcode_line_error(F(STR_ERR_NO_CHECKSUM), p);
             break;
           }