documentclass{article}
usepackage{amsmath}
usepackage{setspace}
usepackage{etoolbox}
usepackage{lipsum}% just to generate text for the example

BeforeBeginEnvironment{equation}{begin{singlespace}}
AfterEndEnvironment{equation}{end{singlespace}noindentignorespaces}
BeforeBeginEnvironment{align}{begin{singlespace}}
AfterEndEnvironment{align}{end{singlespace}noindentignorespaces}

doublespacing

begin{document}

lipsum[2]
begin{equation}
a=b.
end{equation}
lipsum[2]

end{document}

usepackage[nodisplayskipstretch]{setspace}

documentclass{article}
usepackage[nodisplayskipstretch]{setspace}

newcommand{namdui}{Nam dui ligula, fringilla a, euismod sodales,
sollicitudin vel, wisi. Morbi auctor lorem non justo. Nam lacus libero,
pretium at, lobortis vitae, ultricies et, tellus.}  % filler text

doublespacing
begin{document}

namdui
begin{equation}
a=b.
end{equation}
namdui
end{document}

usepackage{setspace}setdisplayskipstretch{}

documentclass[a4paper]{article}  %  For a consistent page count, specify a paper size.
usepackage{setspace}setdisplayskipstretch{}
doublespacing
usepackage{blindtext}  %  Introduces the "Lorem ipsum..." filler text command blindtext
usepackage{pgffor}     %  Introduces the foreach command

begin{document}
  foreach n in {1,...,200}{  % repeat 200 times
    begin{equation}
      a=b.
    end{equation}
    blindtext  % "Lorem ipusm..." filler text
  }
end{document}

╔════════════╦════════════════════════════════════════════════════╗
║ Page count ║ setspace command                                   ║
╠════════════╬════════════════════════════════════════════════════╣
║         77 ║ usepackage{setspace}setdisplayskipstretch{}      ║
║         80 ║ Gonzalo Medina's answer using etoolbox             ║
║         81 ║ usepackage[nodisplayskipstretch]{setspace}        ║
║         86 ║ usepackage{setspace}                              ║
║         93 ║ renewcommand{baselinestretch}{2} % (no setspace) ║
╚════════════╩════════════════════════════════════════════════════╝

newcommand{displayskipstretch}{baselinestretch}
newcommand{setdisplayskipstretch}[1]{renewcommand{displayskipstretch}{#1}}
DeclareOption{nodisplayskipstretch}{setdisplayskipstretch{1.0}}

everydisplayexpandafter{%
  theeverydisplay
  abovedisplayskip displayskipstretchabovedisplayskip
  belowdisplayskip displayskipstretchbelowdisplayskip
  abovedisplayshortskip displayskipstretchabovedisplayshortskip
  belowdisplayshortskip displayskipstretchbelowdisplayshortskip
}