https://tutorial.math.lamar.edu/classes/de/RepeatedRoots.aspx
https://math.libretexts.org/Bookshelves/Analysis/Supplemental_Modules_(Analysis)/Ordinary_Differential_Equations/3%3A_Second_Order_Linear_Differential_Equations/3.1%3A_Homogeneous_Equations_with_Constant_Coefficients