The invention provides systems and methods for making sequencing libraries that are useful for quantitatively analyzing nucleic acids in a sample. Sample nucleic acids are randomly cleaved at, and PCR handled are attached to, a random cut site. The nucleic acid is amplified into a sequencing library in which a sequencing primer generates a sequence read from adjacent the random cut site. The sequence reads can be mapped to a reference, but they will also include a unique identifier sequence that comes from within the nucleic acid molecule being analyzed, i.e., an intrinsic molecular identifier (IMI). The IMI is unique for each molecule and can thus be used to deduplicate sequence reads originating from the same molecule.