WordPress Warnings – Invalid argument supplied (2nd issue – plugin duplication)

I am getting this warning quite often now, how do i fix those. Also see headers already sent and files shows update.php , pluggable.php files.

Warning: Invalid argument supplied for foreach() in /home/deals502/public_html/wp-includes/update.php on line 312

Warning: Invalid argument supplied for foreach() in /home/deals502/public_html/wp-includes/update.php on line 316

one more issue –
I don’t know how it started all of sudden, all installed a plugin which is active now, but also see duplicate same name plugin in the list and i can’t delete it (nothing in ftp folder) even after delete from plugin list it re-appears.