printf() buffers output until a newline is encountered. So your \n-less version stuffs A into the output buffer, forks.
Since both processes are identical (minus PIDs and whatnot), they now both have an output buffer that contains A.
The execution continues and prints B into the buffers in each process. Both processes then exit, causing a flush of the output buffer, which prints AB twice, once for each process.
You other version, having \n, causes that output buffer to flush after the first printf() call, and when the fork hits, both processes have an empty buffer.
