Take some deep breaths before you read this part. Be calm. Meditate. This technical SEO section will pass. Breathe....
Now, are you ready to read some technical SEO?
Because you all know you didn't turn that last website domain off, you didn't redirect the content, you didn't tell your SEO consultant, and now you are wondering why your site is getting penalized. Right? Exactly! (Who said I cannot be passive-aggressive. Just watch and learn, folks.)
Now back on topic. What constitutes "duplicate content"? The short version is that https://example.com and https://example.com/ (note the forward slash at the end of the second example) are NOT duplicate content issues.
But a forward slash at the end of a file name IS duplicate content. And different protocols matter.
1. example.com/fish and example.com/fish/ ARE duplicate content. The former should forward (redirect) to the latter.
2. http://www.example.com, https://www.example.com, http://www.example.com/, and https://www.example.com are ALL DIFFERENT.
Now having written those statements, regarding both #1 and #2, I am going to contradict myself. You can take a deep breath because search engines are generally pretty good at figuring out they are the same thing. (Hello World! Algorithm writing 101.)
Are your eyes rolling yet? Are you asleep? No? Good.
So how do you fix this? Canonical tags, 301 redirects of potential file duplications and protocol duplications to the correct URL, watch/audit your URLs, and check your 404 errors.
I told you, deep breaths, deep breaths, people.
You can also just call me and I will fix it for you***.