  We show that a single $I=1$ spin-parity $J^{PC}= 1^{++}$ $a_1$ resonance can
manifest itself as two separated mass peaks, one decaying into an S-wave
$\rho\pi$ system and the second decaying into a P-wave $f_0(980)\pi$ system,
with a rapid increase of the phase difference between their amplitudes arising
mainly from the structure of the diffractive production process. This study
clarifies questions related to the mass, width, and decay rates of the $a_1$
resonance raised by the recent high statistics data of the COMPASS
collaboration on $a_1$ production in $\pi N \rightarrow \pi \pi \pi N$ at high
energies.
