Use second argument for optional first argument if not provided in macro

There are several ways to do it.

Classical method

\makeatletter
\newcommand{\mycommand}{\@dblarg\my@command}
\def\my@command[#1]#2{\label{#1}#2}
\makeatother

xparse

\usepackage{xparse}
\NewDocumentCommand{\mycommand}{O{#2}m}{\label{#1}#2}

Your use case

\NewDocumentCommand{\mycommand}{O{optionone=#2}m}{%
 \begin{myenv}[#1]\input{#2}\end{myenv}%
}

Here, \mycommand determines if there is 1 or 2 arguments, and passes the result off to \mycommandaux, which handles both arguments, whether repeated or not.

\documentclass{article} 
\newcommand\mycommand[2][\relax]{%
  \ifx\relax#1%
    \mycommandaux{#2}{#2}%
  \else
    \mycommandaux{#1}{#2}%
  \fi
}
\newcommand\mycommandaux[2]{Arguments 1,2: [#1]\{#2\}}
\begin{document}
\mycommand{xyz}

\mycommand[xyz]{pdq}
\end{document} 

Trying to conform more to the OP's edited example

\documentclass{article} 
\usepackage[T1]{fontenc}
\newcommand\mycommand[2][\relax]{%
  \ifx\relax#1%
    \mycommandaux{optionone=#2}{#2}%
  \else
    \mycommandaux{#1}{#2}%
  \fi
}
\newcommand\mycommandaux[2]{\detokenize{\begin{myenv}[#1]\input{#2}\end{myenv}}}
\begin{document}
\mycommand{ABC}

\mycommand[optiontwo=xyz]{pdq}
\end{document}