multiple trajectory outputs with the same "filename" triggers backup

[venkatkapil24]

The following triggers automatic backup. Don't know if this has been discussed before but i-pi should at least trigger a warning if not an error.

    <output prefix="simulation">
        <properties stride="8" filename="out"> [ step, time{picosecond}, conserved, temperature{kelvin}, kinetic_cv, potential, pressure_cv{atmosphere} ] </properties>
        <trajectory filename="pos" stride="8"> velocities </trajectory>
        <trajectory filename="pos" stride="80"> positions </trajectory>
        <checkpoint filename="chk" stride="8000" overwrite="true"/>
    </output>