Coherent units
A system of measurement is coherent when it only contains one unit of distance, one unit of force, one unit of pressure, and so on for each distinct dimension.
The SI system is coherent, since it only has one unit of length (the metre), one unit of time (the second), and so on. In contrast, an “imperial” system containing more than one unit for the same dimension, say both feet and inches for distance, is not coherent.
Redundant names are not redundant units
The SI unit of pressure is the Pascal. However, we can also measure pressure as a force divided by an area: the SI unit of force is the Newton, and the unit for area is the square metre, so “Newtons per square metre” is an SI unit of pressure as well. It appears like SI is not coherent, but in fact the Pascal is just a shorter name for the “Newton per square metre”; that’s how it’s defined.
Likewise, the Newton itself is a shorthand: this time it stands for an SI mass times an SI acceleration. The SI unit of mass is the kilogram (an unfortunate name); whilst an acceleration is distance per squared time, which in SI is metres per square second. Hence the Newton is short for the “kilogram metre per square second”.
It’s fine for a coherent system to give more convenient names to combinations of its existing units; so long those combinations involve no scaling. The metric system is hence not coherent, since it defines scaled units like the litre (0.001 cubic metres), the hour (3600 seconds) and the tonne (1000 kilograms).
Likewise, in imperial systems the foot is not merely another name for a dozen inches: but even if it were, that would be a scaled unit. Many imperial units have been redefined in modern times to be some fixed multiple of the SI equivalent; again, such scaling makes these systems incoherent.
Numerical prefixes are not redundant units
“10 seconds” is not a unit of time distinct from the second, it’s just ten of them. Likewise, a “dozen Volts” is not a unit of potential-difference distinct from the Volt, it’s just twelve of them. Therefore a “kilometre” is not a unit of distance distinct from the metre, it’s just a thousand of them.
Numerical prefixes do not give rise to distinct units.