De-Rating a motor is done to adjust for the altitude as the thinner air effects the cooling of the motor. The higher the altitude the less cooling you will have.
Duty as in Mill duty is a method by which a large motors nameplate is lowered to make it beefier in an attempt to make it last longer and as a cost saving method incase more motor is needed you can squeezes more out of the motor with out having to reorder, and reinstall. Just because you can make something turn, doesn't mean you can stop it the way someone thought you could.
When the motor name plate is changed in this manor the application always involves a DRIVE. This way you can control the torque. By lowering the current limits, operating in the field weaken range, and or limiting the voltage. Motor frame size "generally" determines the current and torque capabilities, while the insulation level and applied voltage determined the horsepower and "generally" speed capabilities. So without a DRIVE controlling the motor you could have enough torque at full speed (60Hz) to shear off a shaft or have fatigue happen over time. Just depends on the machine, man!
When a 60Hz motor is used on a 50Hz line, the motor will run 20% slower and run hotter as it's fixed speed fan turns slower. also the V/Hz ratio will increase so that during part of it's cycle it would be in an overload condition. So, one would need to tap down a transformer (as long as any other equipment would be fine operating at a slightly lower voltage) to compensate.
Running a 50Hz motor on a 60 Hz line, the motor would run ~20% faster causing it to try to operate in it's field weaken range. (I'm sure you know this, however others mint not) When a motor does this HP remains constant and Torque drops. As your application is a pump it has an exponential load curve meaning the faster you turn it the more load you get an the more flow you get too. May be what the one guy was saying, is what you have happening (a larger HP to counter operating in the field weaken range) ... just know no matter what someone changes to nameplate too; more speed equals more flow and higher pressures drops. ( no you don't have to do any thing with a transformer in this case )