Significant progress has been achieved in the classification of topological quasiparticles after the accomplishment of large-scale searches for topological materials. However, some degeneracies in the current classification may actually lie on extended degeneracy manifolds, suggesting that a more refined classification is required. Here, we investigate accidental degeneracies of double bands (ADDBs) along high-symmetry lines in phononic systems and propose a refined classification that distinguishes the charge-0, charge-2 Dirac point, and point in nodal line (PiNL). By analyzing the symmetry constraints through group theory and compatibility relations, we identify all three types of ADDBs (charge-0, charge-2 Dirac point, and PiNL) across all space groups and high-symmetry lines. First-principles calculations on ${\mathrm{FeP}}_2$ confirm the existence of topological nodal-line phonons from ADDBs in realistic phononic materials. This work enriches our understanding of the diverse manifestations of quasiparticle excitations in topological phonon systems.

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• Phonons
• Symmetry protected topological states
• Topological phases of matter
• Dirac semimetal
• Topological materials
• First-principles calculations
• Group theory
• Tight-binding model
• k dot p method