I have two questions regarding GCLID attribution.
2. If a user comes through an ad which has a particular GCLID and then through some other ad, the previous GCLID will get overwritten. Would that be an issue in terms of attribution?
1) You can't delete the cookie. It is Google's cookie on the searcher's device. If you use Google Analytics you can see the complete path and how AdWords is a part of it.
2) Yes, the last ad clicked on will get the credit. Again, using Analytics to see greater detail.
Re: Gclid attribution
There might be a little misunderstanding of the definition of GCLID and cookies.
GCLID (Google Click ID) is a feature, when auto-tagging is enabled, that appends a string of data to your final url that passes information to your Analytics Account (Campaign, Source, Medium, etc) after your ad is clicked such as: www.example?utm_source=Google&utm_medium=CPC&utm_campaign=Search&utm_content=Creative1
That being said, those parameters are set at run time which means if you abandon your session, switch browsers, or switch devices, then those parameter won't be present after the adjustment.
In addition, after a user clicks on your ad, Adwords attaches a cookie on the user's browser. If the user navigated to website destination where the conversion code fires, and the cookie is still present on the user's browser, then that ad will be attributed that conversion.
Depending on how you've setup your attribution settings in adwords (last click model, linear model), often the last ad that was clicked will receive the conversion. But your previous ad can also be attributed a "Click Assisted Conversion".
In summary, GCLID is meant to pass additional data to your Analytics account to monitor and review. The cookies are meant to help associate a unique user with the associated conversion that fires after the goal has been completed.
Hope this helps!