Prolog routes in directed graph - prolog

Question

I am quite new to Prolog and want to create a program, where I can ask for a route with the predicate "route(Startpoint,Endpoint,Route)".
My code so far is:
% facts
connection(s1,s2).
connection(s2,s3).
connection(s3,s4).
connection(s4,s5).
connection(s5,s6).
connection(s6,s7).
connection(s7,s1).
% predicates
route1(X,Y,[X,Y]) :- connection(X,Y).
route1(X,Y,R) :- connection(X,Z), route1(Z,Y,RZ), R=[X|RZ].
route2(X,Y,[X,Y]) :- connection(Y,X).
route2(X,Y,R) :- connection(Z,X), route2(Z,Y,RZ), R=[X|RZ].
route(X,Y,R) :- route1(X,Y,R); route2(X,Y,R).
My code works for some routes, but not when it comes to a cycle (like in the facts above). How can I prevent in Prolog, that a station gets visited in a route multiple times?
For example when I ask "?- route1(s1,s4,R).", Prolog gives me the correct route "[s1,s2,s3,s4]" first, but it also gives me other routes like "[s1, s2, s3, s4, s5, s6, s7, s1, s2, s3, s4]", "[s1, s2, s3, s4, s5, s6, s7, s1, s2, s3, s4, s5, s6, s7, s1, s2, s3, s4]" and so on.
Thanks in advance!

Answer 1

You could just write:
route1(X,Y,[X,Y]) :- connection(X,Y).
route1(X,Y,R) :- connection(X,Z), route1(Z,Y,RZ),R=[X|RZ],
sort(R,R1),length(R,N),length(R1,N1),
(N>N1->!,fail ;true).
sort/2 removes duplicates so if you want your solution not to have duplicates the sorted list and the output list must have same length.
?- route1(s1,s4,R).
R = [s1, s2, s3, s4] ;
false.
Another ways to do that would be :
route1(X,Y,R):-route1(X,Y,[],R).
route1(X,Y,_,[X,Y]) :- connection(X,Y).
route1(X,Y,L,R) :- connection(X,Z),\+member(Z,L),
route1(Z,Y,[Z|L],RZ),R=[X|RZ].
Example:
?- route1(s1,s4,R).
R = [s1, s2, s3, s4] ;
false.