When you copy a configuration to the running configuration, you merge the two configurations. A merge adds any new commands from the new configuration to the running configuration. If the configurations are the same, no changes occur. If commands conflict or if commands affect the running of the context, then the effect of the merge depends on the command. You might get errors, or you might have unexpected results.
If the tftp-server command defines an interface different than the location you wish to copy the a file from, you must use the int keyword to override the interface specified in the tftp-server command, or the device will attempt to copy the file from that interface.