Fatal error: Cannot redeclare admin_notice() [closed]

Ok, solved my own issue… This might be very specific, but in case anyone ever finds a similar issue:

to catch unexpected output errors I had another function:

add_action('activated_plugin','save_error');
function save_error(){
    $file = "unexpected-output.txt";
    $fh = fopen($file, 'w') or die("can't open file");
    $error = ob_get_contents();
    fclose($fh);
}

there was a typo in the fopen(), which apparently messed up things badly (still not sure why it was trying to load everything more than once – I think it has to do with the fact that the function died).