Did you know you could? Yep... here are two instances. The first picture shows the reflection from the window with the shiny tape on it reflecting down on the solar panel as well as the sun. This can give pretty near full boost early in the morning and late in the evening when the panels would barely be doing anything. It can also double the output of panels during the day. No guarantee how much it might shorten the life on the panel though.
The second picture is getting a triple whammy. The sun reflecting from the window and the sun reflecting from the sort of mirror I made. + the sun.