C standard say (6.3.1.1 Boolean, characters, and integers):

1 Every integer type has an integer conversion rank defined as follows:
— No two signed integer types shall have the same rank, even if they hav e the same
representation.
— The rank of a signed integer type shall be greater than the rank of any signed integer
type with less precision.
— The rank of long long int shall be greater than the rank of long int, which
shall be greater than the rank of int, which shall be greater than the rank of short
int, which shall be greater than the rank of signed char.
— The rank of any unsigned integer type shall equal the rank of the corresponding
signed integer type, if any.
— The rank of any standard integer type shall be greater than the rank of any extended
integer type with the same width.
— The rank of char shall equal the rank of signed char and unsigned char.
— The rank of _Bool shall be less than the rank of all other standard integer types.
— The rank of any enumerated type shall equal the rank of the compatible integer type
(see 6.7.2.2).
— The rank of any extended signed integer type relative to another extended signed
integer type with the same precision is implementation-defined, but still subject to the
other rules for determining the integer conversion rank.
— For all integer types T1, T2, and T3, if T1 has greater rank than T2 and T2 has
greater rank than T3, then T1 has greater rank than T3.

This means that in the integer type system there cannot be two types (signed or unsigned) with the same rank. However, further:

The following may be used in an expression wherever an int or unsigned int may
be used:
— An object or expression with an integer type (other than int or unsigned int)
whose integer conversion rank is less than or equal to the rank of int and
unsigned int.

How is it possible that some lesser type will have the same rank?