Left Termination of the query pattern duplicate_in_2(g, a) w.r.t. the given Prolog program could successfully be proven:

(0) Obligation:

Clauses:

duplicate([], L) :- ','(!, eq(L, [])).
duplicate(X, .(H, .(H, Z))) :- ','(head(X, H), ','(tail(X, T), duplicate(T, Z))).
head([], X1).
head(.(H, X2), H).
tail([], []).
tail(.(X3, T), T).
eq(X, X).

Queries:

duplicate(g,a).

(1) PrologToDTProblemTransformerProof (SOUND transformation)

Built DT problem from termination graph.

(2) Obligation:

Triples:

duplicate1(.(T31, T32), .(T31, .(T31, T24))) :- duplicate1(T32, T24).

Clauses:

duplicatec1([], []).
duplicatec1(.(T31, T32), .(T31, .(T31, T24))) :- duplicatec1(T32, T24).

Afs:

duplicate1(x1, x2)  =  duplicate1(x1)

(3) TriplesToPiDPProof (SOUND transformation)

We use the technique of [LOPSTR]. With regard to the inferred argument filtering the predicates were used in the following modes:
duplicate1_in: (b,f)
Transforming TRIPLES into the following Term Rewriting System:
Pi DP problem:
The TRS P consists of the following rules:

DUPLICATE1_IN_GA(.(T31, T32), .(T31, .(T31, T24))) → U1_GA(T31, T32, T24, duplicate1_in_ga(T32, T24))
DUPLICATE1_IN_GA(.(T31, T32), .(T31, .(T31, T24))) → DUPLICATE1_IN_GA(T32, T24)

R is empty.
The argument filtering Pi contains the following mapping:
duplicate1_in_ga(x1, x2)  =  duplicate1_in_ga(x1)
.(x1, x2)  =  .(x1, x2)
DUPLICATE1_IN_GA(x1, x2)  =  DUPLICATE1_IN_GA(x1)
U1_GA(x1, x2, x3, x4)  =  U1_GA(x1, x2, x4)

We have to consider all (P,R,Pi)-chains

Infinitary Constructor Rewriting Termination of PiDP implies Termination of TRIPLES

(4) Obligation:

Pi DP problem:
The TRS P consists of the following rules:

DUPLICATE1_IN_GA(.(T31, T32), .(T31, .(T31, T24))) → U1_GA(T31, T32, T24, duplicate1_in_ga(T32, T24))
DUPLICATE1_IN_GA(.(T31, T32), .(T31, .(T31, T24))) → DUPLICATE1_IN_GA(T32, T24)

R is empty.
The argument filtering Pi contains the following mapping:
duplicate1_in_ga(x1, x2)  =  duplicate1_in_ga(x1)
.(x1, x2)  =  .(x1, x2)
DUPLICATE1_IN_GA(x1, x2)  =  DUPLICATE1_IN_GA(x1)
U1_GA(x1, x2, x3, x4)  =  U1_GA(x1, x2, x4)

We have to consider all (P,R,Pi)-chains

(5) DependencyGraphProof (EQUIVALENT transformation)

The approximation of the Dependency Graph [LOPSTR] contains 1 SCC with 1 less node.

(6) Obligation:

Pi DP problem:
The TRS P consists of the following rules:

DUPLICATE1_IN_GA(.(T31, T32), .(T31, .(T31, T24))) → DUPLICATE1_IN_GA(T32, T24)

R is empty.
The argument filtering Pi contains the following mapping:
.(x1, x2)  =  .(x1, x2)
DUPLICATE1_IN_GA(x1, x2)  =  DUPLICATE1_IN_GA(x1)

We have to consider all (P,R,Pi)-chains

(7) PiDPToQDPProof (SOUND transformation)

Transforming (infinitary) constructor rewriting Pi-DP problem [LOPSTR] into ordinary QDP problem [LPAR04] by application of Pi.

(8) Obligation:

Q DP problem:
The TRS P consists of the following rules:

DUPLICATE1_IN_GA(.(T31, T32)) → DUPLICATE1_IN_GA(T32)

R is empty.
Q is empty.
We have to consider all (P,Q,R)-chains.

(9) QDPSizeChangeProof (EQUIVALENT transformation)

By using the subterm criterion [SUBTERM_CRITERION] together with the size-change analysis [AAECC05] we have proven that there are no infinite chains for this DP problem.

From the DPs we obtained the following set of size-change graphs:

• DUPLICATE1_IN_GA(.(T31, T32)) → DUPLICATE1_IN_GA(T32)
  The graph contains the following edges 1 > 1

(10) YES