What causes double slashes in url path?

Question

I have a lot of problem with Google search, because it (stochastically) finds many duplicates in the form of

http://www.example.com/path/filename.html
http://www.example.com/path//filename.html

I have checked all files throughout and I can't find any error. Does anyone knows any trick how to catch this error or knows what unobvious error could cause it?

I have written the whole site in pure HTML myself.

EDIT Following the suggestion from Stephen Ostermiller, I am sharing my .htaccess

# remove index.html
RewriteCond %{THE_REQUEST} ^.*\/index\.html?
RewriteRule ^(.*)index\.html?$ https://www.example.com/$1 [R=301,L]
# add www.
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ https://www.%{HTTP_HOST}/$1 [R=301,L]
# enforce https
RewriteCond %{HTTPS} !on
RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [R=301,L]

Answer 1

Double slashes are usually caused by redirect rules. For example RewriteRule ^(.*)$ http://www.example.com/path/$1 [R=301,L] could cause the double slash. The regular expression's parenthesis might be returning /filename.html for the $1 variable. The two slashes come from one from the match and the one hard coded.

To make this redirect work as expected, you could have the rule start with an optional slash that isn't captured: RewriteRule ^/?(.*)$ http://www.example.com/path/$1 [R=301,L] Then the captured group wouldn't contain the leading slash. Alternately, you could remove the second hard coded slash and rely on the captured group having the slash: RewriteRule ^(.*)$ http://www.example.com/path$1 [R=301,L].

I prefer the first of those two solutions. Rewrite rules match leading slashes differently depending on which context they are used in. The URL path does not have a leading slash when rewrite rules are used in .htaccess but the path does have a leading slash when the rewrite rules are used in httpd.conf. Making the leading slash optional with ^/? allows the rule to work correctly in either context.

The problem is compounded by relative links once a crawler finds a URL with a duplicate slash. If http://www.example.com/path//filename.html contains a link like <a href="otherfile.html"> the the crawler will also fetch http://www.example.com/path//otherfile.html. Crawlers will find fewer duplicate slashes on your site if you use "root relative links" on your site. Root relative links start with a slash and link relative to your domain name. If the link were <a href="/path/otherfile.html"> then the crawler wouldn't find additional duplicate slash URLs from one malformed URL.

Once a crawler has found URLs with duplicate slashes, it may continue to crawl them indefinitely, even after you fix all the redirect rules and links. It is a good idea to add rules to remove double slashes from URLs. It looks like this rule set from Issue In Removing Double Or More Slashes From URL By .htaccess is a good one:

# rule 1: remove multiple leading slashes (directly after the TLD)
RewriteCond %{THE_REQUEST} \s/{2,}
RewriteRule (.*) $1 [R=301,L]

# rule 2: remove multiple slashes in the requested path
RewriteCond %{REQUEST_URI} ^(.*)/{2,}(.*)$
RewriteRule (.*) %1/%2 [R=301,L]