  We investigate the general interaction between an eccentric planet and a
coplanar debris disc of the same mass, using analytical theory and n-body
simulations. Such an interaction could result from a planet-planet scattering
or merging event. We show that when the planet mass is comparable to that of
the disc, the former is often circularised with little change to its semimajor
axis. The secular effect of such a planet can cause debris to apsidally
anti-align with the planet's orbit (the opposite of what may be naively
expected), leading to the counter-intuitive result that a low-mass planet may
clear a larger region of debris than a higher-mass body would. The interaction
generally results in a double-ringed debris disc, which is comparable to those
observed in HD 107146 and HD 92945. As an example we apply our results to HD
107146, and show that the disc's morphology and surface brightness profile can
be well-reproduced if the disc is interacting with an eccentric planet of
comparable mass (~10-100 Earth masses). This hypothetical planet had a
pre-interaction semimajor axis of 30 or 40 au (similar to its present-day
value) and an eccentricity of 0.4 or 0.5 (which would since have reduced to
~0.1). Thus the planet (if it exists) presently resides near the inner edge of
the disc, rather than between the two debris peaks as may otherwise be
expected. Finally we show that disc self-gravity can be important in this mass
regime and, whilst it would not affect these results significantly, it should
be considered when probing the interaction between a debris disc and a planet.
