fixed #70 + made db script ignore double bookings

Backend/models/booking.go

@@ -128,6 +128,9 @@ func (b *Booking) InsertWithTimestamp() error {
 	if b.Timestamp.IsZero() {
 		return b.Insert()
 	}
+	if !checkLastBooking(*b) {
+		return SameBookingError{}
+	}
 	stmt, err := DB.Prepare((`INSERT INTO anwesenheit (card_uid, geraet_id, check_in_out, anwesenheit_typ, timestamp) VALUES ($1, $2, $3, $4, $5) RETURNING counter_id`))
 	if err != nil {
 		return err