It looks like a linear plating bubble that just happens to look like an extra upright for the 9. If you take a wooden toothpick and press on it you may find it to be flexible. It's not doubling or a strike error. It's common on copper plated zinc coins.
"Shine, shine a Roosevelt dime. All the way to Baltimore and runnin' out of time." Tom Waits-Clap Hands