The following code complains if NewSize is set the same as InitialHeapSize (or -Xms):
  // Make sure NewSize allows an old generation to fit even if set on the command line
  if (FLAG_IS_CMDLINE(NewSize) && NewSize >= InitialHeapSize) {
    log_warning(gc, ergo)("NewSize was set larger than initial heap size, will use initial heap size.");
    FLAG_SET_ERGO(NewSize, bound_minus_alignment(NewSize, InitialHeapSize, GenAlignment));
  }

This is the output of a few runs:

$ ../build/fastdebug/jdk/bin/java -Xlog:gc+init,heap*=trace,exit*=off -XX:+UseSerialGC -Xmx256m -Xms128m -XX:NewSize=128m -version
```
[0.012s][info][gc,init] CardTable entry size: 512
[0.012s][warning][gc,ergo] NewSize was set larger than initial heap size, will use initial heap size.
[0.012s][debug ][gc,heap] Minimum heap 134217728 Initial heap 134217728 Maximum heap 268435456
[0.012s][trace ][gc,heap] 1: Minimum young 134152192 Initial young 134152192 Maximum young 134152192
[0.012s][trace ][gc,heap] Minimum old 65536 Initial old 65536 Maximum old 134283264
```
$ ../build/fastdebug/jdk/bin/java -Xlog:gc+init,heap*=trace,exit*=off -XX:+UseSerialGC -Xmx256m -Xms128m -XX:NewSize=134217728 -version
```
[0.012s][info][gc,init] CardTable entry size: 512
[0.012s][warning][gc,ergo] NewSize was set larger than initial heap size, will use initial heap size.
[0.012s][debug ][gc,heap] Minimum heap 134217728 Initial heap 134217728 Maximum heap 268435456
[0.012s][trace ][gc,heap] 1: Minimum young 134152192 Initial young 134152192 Maximum young 134152192
[0.012s][trace ][gc,heap] Minimum old 65536 Initial old 65536 Maximum old 134283264
```
$ ../build/fastdebug/jdk/bin/java -Xlog:gc+init,heap*=trace,exit*=off -XX:+UseSerialGC -Xmx256m -Xms128m -XX:NewSize=134217727 -version
```
[0.019s][info][gc,init] CardTable entry size: 512
[0.019s][debug][gc,heap] Minimum heap 134217728 Initial heap 134217728 Maximum heap 268435456
[0.019s][trace][gc,heap] 1: Minimum young 134152192 Initial young 134152192 Maximum young 134152192
[0.019s][trace][gc,heap] Minimum old 65536 Initial old 65536 Maximum old 134283264
```

It is unclear if there's an actual problem with the code, or if this is just a very misleading warning message.