We present a simple method to develop a compact, reliable, and robust free-running erbium single-cavity dual-comb (DC) laser via polarization multiplexing. The key features of our design include dynamic tuning in the difference in repetition rates of the laser, long-term stability, and the use of off-the-shelf components. Polarization multiplexing exploits the fast and slow axes of the fiber, while modelocking is achieved through a nonlinear amplifying loop mirror scheme using readily available components. The laser operates at a repetition rate of around 74.74 MHz with a tuning capability in the difference in repetition rates from 500 Hz to 200 kHz. This tunability makes the system more flexible for DC spectroscopy experiments. Consequently, using this laser, we demonstrated a proof-of-principle DC spectroscopy of carbon monoxide, operating without any active stabilization.