Double-pane windows, also called "insulated" or "thermal" windows, are made with two panels of glass sandwiched over a spacer and sealant, creating a space between the panes that is filled with air or with a gas, usually argon or krypton. It is this sealed space that gives the window most of it's thermal performance. The spacer may also contain a desiccant material to help absorb and moisture in the air within the sealed space. Condensation in a thermal window typically indicates a failure of the seal between the glass panels and/or saturation of the desiccant.
Because a breached seal allows fresh air and water vapor to enter the window space, a condensation problem simply becomes a function of nature. Condensation is also a sure sign that gas-filled windows are no longer so. And since there's no way to re-seal an insulated window, replacement of the glass is the only option for restoring maximum thermal performance, in addition to solving the condensation problem. As mentioned, most glazing on insulated windows can be replaced as a unit, which saves you the expense and remodeling work of replacing the entire window, frame and all.
Reference article: http://www.networx.com/article/how-to-fix-condensation-in-double-pane-w