I faced same issue when Publisher was on SQL Server 2012 SP1 and distributor was on SQL server 2008 SP3 and is resolved by updating distributor to SQL Server 2012...

SQL environment should be kept same for each execution.First time you run the query ,query plan is cached .Next time it will use that query plan.

Time also depends upon statistics...

What is the query ? Post it and let's finish the discussion

When you say duplicate, does it mean that duplicate values exist for all columns or for certain columns ? Depending upon number of columns having duplicate values, those columns should...

On way could be that restore previous backup without errors on test box and compere contents of corrupt table with that tbale and you may pin point your data loss..

The mirrored database is in resttoring mode and you cannot make changes.

I think to a extend you need to understand coupling between modules and the module you are looking for optimisation.

Mostly optimisation ends with database alteration as mostly database suffer with...