Re-wet Edge of Stain
Unlike painting you cannot fix this by painting/staining over it seems. The only working solution is to remove stain somewhat uniformly from the overlapping areas using thinner. I'm not sure for how long after staining you can still use thinner. If that stops working, you can try paint/varnish strippers, which may remove some but not all staining because stain sinks into the wood fibers rather than forming a film like paint. Failing all that, you have to sand the wood to remove the marks.