C (gcc), 168 160 119 bytes

-8 Thanks to @corvus_192 for pointing out newline separation between values was allowed, and a massive -41 thanks to @tsh

f(l,n,c,i)char*l;char*c;{if(!n)puts(c);else for(int j=0;j<strlen(l);j++)if(!i||c[i-1]^l[j])c[i]=l[j],f(l,n-1,c,i+1);}g(l,n)char*l;{char c[n+1];f(l,n,c,c[n]=0);}

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Prints the newline separated output to STDOUT.

C (gcc), 168 160 119 114 bytes

-8 Thanks to @corvus_192 for pointing out newline separation between values was allowed, and a massive -41 thanks to @tsh, a further -5 thanks to @ceilingcat

char*l,*c;f(n,i){n?(c[n-1]=l[i])&&(l[i]-c[n]&&f(n-1,0),f(n,i+1)):puts(c);}g(e,n)char*e;{c=calloc(n,2);l=e;f(n,0);}

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Prints the newline separated output to STDOUT.

C (gcc), 168 160 bytes

-8 Thanks to corvus_192 for pointing out newline separation between values was allowed

f(l,n,c,i)char*l;char*c;{if(!n)puts(c);else for(int j=0;j<strlen(l);j++)if(!i||c[i-1]^l[j])c[i]=l[j],f(l,n-1,c,i+1);}g(l,n)char*l;{char c[n+1];f(l,n,c,c[n]=0);}

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Prints the newline separated output to STDOUT.

It builds a n+1 sized char array, null terminates it in preparation for it being used to build the strings, then iterates recursively over l ignoring cases where the current index of our char array is equal to the character being iterated on in l.

C (gcc), 168 160 119 bytes

-8 Thanks to @corvus_192 for pointing out newline separation between values was allowed, and a massive -41 thanks to @tsh

f(l,n,c,i)char*l;char*c;{if(!n)puts(c);else for(int j=0;j<strlen(l);j++)if(!i||c[i-1]^l[j])c[i]=l[j],f(l,n-1,c,i+1);}g(l,n)char*l;{char c[n+1];f(l,n,c,c[n]=0);}

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Prints the newline separated output to STDOUT.

C (gcc), 168 160 bytes

-8 Thanks to corvus_192 for pointing out newline separation between values was allowed

f(l,n,c,i)char*l;char*c;{if(!n)puts(c);else for(int j=0;j<strlen(l);j++)if(!i||c[i-1]^l[j])c[i]=l[j],f(l,n-1,c,i+1);}g(l,n)char*l;{char c[n+1];f(l,n,c,c[n]=0);}

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Prints the space separated output to STDOUT with a trailing space but no newline.

It builds a n+1 sized char array, null terminates it in preparation for it being used to build the strings, then iterates recursively over l ignoring cases where the current index of our char array is equal to the character being iterated on in l.

C (gcc), 168 160 bytes

-8 Thanks to corvus_192 for pointing out newline separation between values was allowed

f(l,n,c,i)char*l;char*c;{if(!n)puts(c);else for(int j=0;j<strlen(l);j++)if(!i||c[i-1]^l[j])c[i]=l[j],f(l,n-1,c,i+1);}g(l,n)char*l;{char c[n+1];f(l,n,c,c[n]=0);}

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Prints the newline separated output to STDOUT.

It builds a n+1 sized char array, null terminates it in preparation for it being used to build the strings, then iterates recursively over l ignoring cases where the current index of our char array is equal to the character being iterated on in l.