  Fast radio bursts (FRBs) are brilliant short-duration flashes of radio
emission originating at cosmological distances. The vast diversity in the
properties of currently known FRBs, and the fleeting nature of these events
make it difficult to understand their progenitors and emission mechanism(s).
Here we report high time resolution polarization properties of FRB 20210912A, a
highly energetic event detected by the Australian Square Kilometre Array
Pathfinder (ASKAP) in the Commensal Real-time ASKAP Fast Transients (CRAFT)
survey, which show intra-burst PA variation similar to Galactic pulsars and
unusual variation of Faraday Rotation Measure (RM) across its two sub-bursts.
The observed intra-burst PA variation and apparent RM variation pattern in FRB
20210912A may be explained by a rapidly-spinning neutron star origin, with
rest-frame spin periods of ~1.1 ms. This rotation timescale is comparable to
the shortest known rotation period of a pulsar, and close to the shortest
possible rotation period of a neutron star. Curiously, FRB 20210912A exhibits a
remarkable resemblance with the previously reported FRB 20181112A, including
similar rest-frame emission timescales and polarization profiles. These
observations suggest that these two FRBs may have similar origins.
