Division by zero error on line 78 of gpx.inc

Sometimes I get division by zero errors when using geocode with filefield gpx file uploads.

The cause is having two points in a row that are the same so the miles per hour calculation gives an error.

I assume consecutive duplicate points might not be uncommon with some gps devices.

This patch makes gpx.inc skip consecutive duplicate points. There is no need for them in a line string anyway.