scp is a Unix command and is included in most Unix and Unix-based operating systems such as Linux and macOS. If you are using a Windows system, you will need to use a different tool such as Putty or WinSCP to transfer files between servers. These tools work similarly to scp and provide a graphical interface for copying files over SSH.

Regarding your second question, by default scp will overwrite existing files in the target directory. However, you can use the "-n" option to avoid overwriting files and to preserve the existing files in the target directory. For example:  
scp -rn user_name@server_A:~/Documents/training_results user_name@server_B:~/training_results
spam fails_task lang_mismatch pii not_appropriate hate_speech sexual_content quality toxicity humor helpfulness creativity violence