What is Canonical issue?

canonicalization refers to individual web pages that can be accessed from multiple URLs. This is because when different pages having same Content but multiple URLs, links that are pointed to go to the same page but itself split into multiple URLs.
For example if your website http://www.example.com are also accessed from different URLs like
http://example.com
http://example.com/index.html
http://www.example.com/index.html

Different Methods are there to resolve Canonical issue Problems

1.Using Google Webmaster Tool

Go to Google Webmaster tool
Login to http://example.com which having Canonical issue
In setting toolbar at the top right > select site setting
In site settings select the preferred domain as www or non www which google like to use.

2.301 Redirect

There are two ways to set 301 redirect

a..htacess

To do 301 permanent redirect simply paste the following below code in .htaccess file of your root directory or copy the code in notepad and save it as .htaccess & place in the root directory of the domain

Options +FollowSymLinks
RewriteEngine on
RewriteCond %{HTTP_HOST} ^ example.com [NC]
RewriteRule ^(.*)$ http://www.example .com/$1 [L,R=301]

RewriteCond %{REQUEST_URI} ^(.*)//(.*)$
RewriteRule . http://www.example.com%1/%2 [R=301,L]
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /(([^/]+/)*)index\.html\ HTTP/
RewriteRule index\.html$ http://www.example.com/%1 [R=301,L]\

In the above code you need to change the example.com as your domain name

In this place in the above code

index\.html\ HTTP/
index\.html$

.html as .php index page it depends on website that you have created in php or html or may not be index. check with your website and change in that way.

b.setting up 301 permanent redirect using windows server

If you hosted your website on a windows server you need to have administrative access to that hosting server and set up 301 through IIS.

Go to All Programs-Administrative Tools-Internet Information Services
Select your domain name - right click- select properties
In that select home directory tab
Click on the radial button - A redirection to a URL
Then enter the URL you want to redirect
Click Ok

Then your domain is redirected.

c.Rel Canonical Tag

If duplicate content issues arise in your domain webpages. You can solve this by including this code in your html page header section

<link rel=”canonical” href=”https://www.example.com” />