  In this paper, we investigated the theoretical and cosmological effects of
the matter Lagrangian degeneracy in an extension of the Symmetric Teleparallel
Equivalent of General Relativity, denoted as $f (Q, T )$ gravity. This
degeneracy comes from the fact that both $\mathcal{L}_m = p$ and $\mathcal{L}_m
= -\rho$ can give rise to the stress-energy tensor of a perfect fluid. The
$f(Q,T)$ equations depend on the form of the matter Lagrangian and hence they
also have a degeneracy that has influence on the density evolution of dust
matter and radiation, on the form of the generalized Friedmann equations and
also shows changes in the cosmological parameters confidence regions when
performing Monte Carlo Markov Chains analyses. Our results suggest that since
changing the matter Lagrangian causes different theoretical and cosmological
results, future studies in $f(Q,T)$ gravity should consider both forms of the
matter Lagrangians to account for the different mathematical and observational
results.
