Most block ciphers iterate the same round function, but this regularity is destroyed by using distinct round keys in each round.
The only vulnerabilities of the iterated construction appear when a weak method is used for generating the round keys from the cipher key (i.e. when the so-called key schedule is weak), so that there are predictable relationships between the round keys.
There exists an alternative (and equivalent) construction for a block cipher, when the same key is introduced in all rounds, but in this case all the round functions must be different from each other (instead of iterating the same function).
The only vulnerabilities of the iterated construction appear when a weak method is used for generating the round keys from the cipher key (i.e. when the so-called key schedule is weak), so that there are predictable relationships between the round keys.
There exists an alternative (and equivalent) construction for a block cipher, when the same key is introduced in all rounds, but in this case all the round functions must be different from each other (instead of iterating the same function).