  The spectrum of a quasar contains important information about its properties.
Thus, it can be expected that two quasars with similar spectra will have
similar properties, but just how similar has not before been quantified. Here
we compare the ultraviolet spectra of a sample of 5553 quasars from Data
Release 7 of the Sloan Digital Sky Survey, focusing on the $1350$ \AA \ $\leq
\lambda \leq 2900$ \AA \ rest-frame region which contains prominent emission
lines from \SiIV, O IV], \CIV, \CIII, and \MgII\ species. We use principal
component analysis to determine the dominant components of spectral variation,
as well as to quantitatively measure spectral similarity. As suggested by both
the Baldwin effect and modified Baldwin effect, quasars with similar spectra
have similar properties: bolometric luminosity, Eddington fraction, and black
hole mass. The latter two quantities are calculated from the luminosity in
conjunction with spectral features, and the variation between quasars with
virtually identical spectra (which we call doppelg\"angers) is driven by the
variance in the luminosity plus measurement uncertainties. In the doppelgangers
the luminosity differences show 1$\sigma$ uncertainties of 57\% (or 0.63
magnitudes) and $\sim$70\% 1$\sigma$ uncertainties for mass and Eddington
fraction. Much of the difference in luminosities may be attributable to time
lags between the spectral lines and the continuum. Furthermore, we find that
suggestions that the mostly highly accreting quasars should be better standard
candles than other quasars are not bourne out for doppelgangers. Finally, we
discuss the implications for using quasars as cosmological probes and the
nature of the first two spectral principal components.
