  Twinlike defects refer to topological defect solutions of some apparently
different field models that share the same defect configuration and the same
energy density. Usually, one can distinguish twinlike defects in terms of their
linear spectra, but in some special cases twinlike defects even share the same
linear spectrum. In this paper, we derive the algebraic conditions for two
twinlike defects to share identical linear spectrum from the viewpoint of the
normal modes of the linear fluctuations. We also extend our discussion to
braneworld models, where gravity plays an important role.
