R, 94 92 bytes

function(s,n=nchar(s),g=substring)if(n)g(s,d<-which(!duplicated(g(s,1:n,1:n))),c(d[-1]-1,n))

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Returns a (possibly empty) list of substrings.

Thanks to Michael M for saving 2 bytes!

R, 94 87 bytes

function(s,n=nchar(s),g=substring)g(s,d<-which(!duplicated(g(s,1:n,1:n))),c(d[-1]-1,n))

Try it online!

Returns a (possibly empty) list of substrings.

Thanks to Michael M for saving 7 bytes!

R, 94 87 bytes

function(s,n=nchar(s),g=substring)g(s,d<-which(!duplicated(g(s,1:n,1:n))),c(d[-1]-1,n))

Try it online!

Returns a (possibly empty) list of substrings.

Thanks to Michael M for saving 7 bytes!

R, 94 92 bytes

function(s,n=nchar(s),g=substring)if(n)g(s,d<-which(!duplicated(g(s,1:n,1:n))),c(d[-1]-1,n))

Try it online!

Returns a (possibly empty) list of substrings.

Thanks to Michael M for saving 2 bytes!

R, 94 bytes

function(s,n=nchar(s))if(n)substring(s,d<-which(!duplicated(el(strsplit(s,"")))),c(d[-1]-1,n))

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Returns a (possibly empty) list of substrings.

R, 94 87 bytes

function(s,n=nchar(s),g=substring)g(s,d<-which(!duplicated(g(s,1:n,1:n))),c(d[-1]-1,n))

Try it online!

Returns a (possibly empty) list of substrings.

Thanks to Michael M for saving 7 bytes!