#Pyth, 27 26 bytes

&zhfTmf!/>zhxzYYm<>zkdUzUz

Try it here.

Note that due to a bug in the online compiler, the empty string case only works correctly on the command line version, which can be found here.

You can also cure the bug by giving a newline as the input for the online compiler.

Explanation:

                                   z = input(), implicit.
&z                                 Prints empty string if input is empty.
  hfT                              Take the first non-empty list from
     m                  Uz         A list of list of substrings of z, divided by length
                m<>zkdUz           with some shorter strings repeated later, to no effect.
      f                            Where the substrings are filtered on
       !/      Y                   There being 0 occurrences of the substring in
         >z                        The slice of z
           hxzY                    from the character after the first character
                                   of the first occurrence of the substring in z
                                   to the end of z.

Pyth, 27 26 bytes

&zhfTmf!/>zhxzYYm<>zkdUzUz

Try it here.

Note that due to a bug in the online compiler, the empty string case only works correctly on the command line version, which can be found here.

You can also cure the bug by giving a newline as the input for the online compiler.

Explanation:

                                   z = input(), implicit.
&z                                 Prints empty string if input is empty.
  hfT                              Take the first non-empty list from
     m                  Uz         A list of list of substrings of z, divided by length
                m<>zkdUz           with some shorter strings repeated later, to no effect.
      f                            Where the substrings are filtered on
       !/      Y                   There being 0 occurrences of the substring in
         >z                        The slice of z
           hxzY                    from the character after the first character
                                   of the first occurrence of the substring in z
                                   to the end of z.

#Pyth, 27 26 bytes

&zhfTmf!/>zhxzYYm<>zkdUzUz

Try it here.

Note that due to a bug in the online compiler, the empty string case only works correctly on the command line version, which can be found here.

#Pyth, 27 26 bytes

&zhfTmf!/>zhxzYYm<>zkdUzUz

Try it here.

Note that due to a bug in the online compiler, the empty string case only works correctly on the command line version, which can be found here.

You can also cure the bug by giving a newline as the input for the online compiler.

Explanation:

                                   z = input(), implicit.
&z                                 Prints empty string if input is empty.
  hfT                              Take the first non-empty list from
     m                  Uz         A list of list of substrings of z, divided by length
                m<>zkdUz           with some shorter strings repeated later, to no effect.
      f                            Where the substrings are filtered on
       !/      Y                   There being 0 occurrences of the substring in
         >z                        The slice of z
           hxzY                    from the character after the first character
                                   of the first occurrence of the substring in z
                                   to the end of z.

#Pyth, 27 26 bytes

&zhfTmf!/>zhxzYYm<>zkdUzUz

Try it here.

Explanation to follow.

#Pyth, 27 26 bytes

&zhfTmf!/>zhxzYYm<>zkdUzUz

Try it here.

Note that due to a bug in the online compiler, the empty string case only works correctly on the command line version, which can be found here.