How you can add Conditional jquery in wordpress if theme haven't jquery. I've wordpress plugin designed to use jquery to operate the main problem is that if i include jquery in wordpress plugin and theme already incorporated or hardcoded in header jquery so 2times will jquery include header.

Same with their in whatever way if jquery present in theme then wordpress plugin jquery not include.

I'd use wp_enqueue_script pointed out above and rewrote theme header and wordpress plugin. That could save you plenty of support amount of time in future.

Do this script:

<script>window.jQuery || document.write('<script src="js/libs/jquery.js">\x3C/script>')