When the command line exceeds 1024 characters RDMD will use an rsp file, which should only contain flags for the compiler. But RDMD puts the actual compiler in the file as well.
https://github.com/D-Programming-Language/tools/pull/3 https://github.com/D-Programming-Language/tools/pull/8
*** Issue 6543 has been marked as a duplicate of this issue. ***
Fixed in 2.055 via: https://github.com/D-Programming-Language/tools/pull/3