Check for duplicate dihedrals in the PSF files

[resal81]

From here:

There is a difference between X-PLOR formatted CHARMM PSF files, which VMD generates with the psfgen module and NAMD uses in conjunction with CHARMM parameter files, and PSF files generated by X-PLOR, which NAMD uses in conjunction with X-PLOR parameter files. Dihedral terms sometimes require multiple sinusoids to represent a torsional potential and therefore multiple parameter sets appear in the parameter file. In PSF files generated by X-PLOR multiple dihedrals would be indicated by duplicate dihedrals in the PSF file. When using CHARMM PSF and parameter files NAMD extracts any multiple dihedral terms directly from the parameter file and each dihedral appears only once in the PSF file.