You could copy the internal state of the chain using a copy agent C(o[n.E], c[n.E]) and some Kappa rules.
E(l, n{A}, d{b}) -> E(l, n{A}[1], d{b}), C(o[1], c[2]), E(n{A}[2], d{b})
E(l, n{B}, d{f}) -> E(l, n{B}[1], d{f}), C(o[1], c[2]), E(n{B}[2], d{f})
E(l, n{B}, d{b}) -> E(l, n{B}[1], d{b}), C(o[1], c[2]), E(n{B}[2], d{b})
E(l, n{C}, d{f}) -> E(l, n{C}[1], d{f}), C(o[1], c[2]), E(n{C}[2], d{f})
E(l, n{C}, d{b}) -> E(l, n{C}[1], d{b}), C(o[1], c[2]), E(n{C}[2], d{b})
# copy next monomer
E(r[1], n[2]), E(l[1], n{A}, d{f}), C(o[2], c[3]), E(r, n[3]) -> E(r[1], n), E(l[1], n{A}[2], d{f}), C(o[2], c[5]), E(r[4], n), E(l[4], n{A}[5], d{f})
E(r[1], n[2]), E(l[1], n{A}, d{b}), C(o[2], c[3]), E(r, n[3]) ->
E(r[1], n), E(l[1], n{A}[2], d{b}), C(o[2], c[5]), E(r[4], n), E(l[4],
n{A}[5], d{b})
E(r[1], n[2]), E(l[1], n{B}, d{f}), C(o[2], c[3]), E(r, n[3]) ->
E(r[1], n), E(l[1], n{B}[2], d{f}), C(o[2], c[5]), E(r[4], n), E(l[4],
n{B}[5], d{f})
E(r[1], n[2]), E(l[1], n{B}, d{b}), C(o[2], c[3]), E(r, n[3]) ->
E(r[1], n), E(l[1], n{B}[2], d{b}), C(o[2], c[5]), E(r[4], n), E(l[4],
n{B}[5], d{b})
E(r[1], n[2]), E(l[1], n{C}, d{f}), C(o[2], c[3]), E(r, n[3]) ->
E(r[1], n), E(l[1], n{C}[2], d{f}), C(o[2], c[5]), E(r[4], n), E(l[4],
n{C}[5], d{f})
E(r[1], n[2]), E(l[1], n{C}, d{b}), C(o[2], c[3]), E(r, n[3]) ->
E(r[1], n), E(l[1], n{C}[2], d{b}), C(o[2], c[5]), E(r[4], n), E(l[4],
n{C}[5], d{b})
# finish copying
E(r, n[1]), E(n[2]), C(o[1], c[2]) -> E(r, n), E(n)
I recommend you draw them to see more easily what these rules are doing. With these rules you can copy chains of any length.