Alias vs Duplicate
In Mac OS, ‘duplicate’ and ‘alias’ are functions that both involve similar, if not one and the same, sets of files.  However synonymous these two commands may sound, they are different in terms of technical function, creation, and practical application.
By definition, ‘duplicate’ is a noun that pertains to a copy that exactly corresponds to an original or a verb that means to double, repeat or perform again. It means pretty much the same with the MAC OS function. Duplicate simply makes a copy of a file in the same location as the original. It shouldn’t however be mistaken for ‘copy’ function. Copy creates an identical file but instead of storing it in the same location, it puts it in the clipboard. Alias, a noun which means ‘assumed name’ or verb meaning ‘also known as’, creates a new small file or folder that links to the original. It is similar to the concept of shortcut in Windows. Instead of being a physical copy of a new file, it is only an icon that point to the actual file, thus providing a much more convenient access.
One can create a duplicate on the same disk by selecting an item- can be a file or folder- and choosing File, then Duplicate. It then creates an exact copy of the chosen file; however no two items can share the same name in the same location. The duplicate, therefore, will have an appendage ‘copy’ to its file name. The duplicate can be moved to another folder and be assigned a different name. As for the alias command, it can be created by going to the Finder, selecting the file for which to create an alias, and then choosing File, and lastly Make Alias. An alias is distinguished by the word ‘alias’ attached to the icon name. Like in the case of a duplicate file or folder, it can be moved to different location as preferred.
A duplicate item loses any connection or relation to the original one. This means that if one is edited, renamed, or shared, the other would not in any way be affected. Deleting a duplicate doesn’t also erase the original and vice-versa. There’s a different scenario for an alias. Since an alias is a mere link to the original file, whatever alteration made to it will also apply to the original file or folder. Deleting an alias icon won’t do any harm to the original file. What’s actually being deleted is just the link.
An alias is a practical way to access files, especially those that are rather difficult to take hold of. For instance, if one has a file buried 4 folders deep on the hard disk; he can save the hassle by simply creating an alias on the desktop. This then allows him to access the original file in just a click. In addition, the size of an alias is so insignificant; it barely eats up disk space. Conversely, a duplicate nearly consumes as much space as the original item does. Creating multiple duplicates can only fill up the Mac’s hard disk.
Summary

1. Duplicate and alias are functions used in Mac Operating Systems.
2. Duplicate creates a physical copy of the original- the same content, the same size-while alias creates a link icon that points to the original file. Both duplicate and alias files can be moved to a different location as preferred.
3. A duplicate loses any relation to the original file, while an alias is always relative to the actual except for instances of deletion.
4. A duplicate file or folder eats up as much space as the original file. An alias is insignificant in size.