http://www.newsweek.com/instagram-repost-how-app-iphone-android-1174135

Twitter has the retweet option and Facebook has the option to share posts, but Instagram doesn’t have the option to take someone else's post and share it to a user’s own personal feed. The option to “repost” on the app isn’t ingrained in the design, so users have had to get creative or use third-party apps to share what they want.

Sometimes those third-party apps add a watermark or some type of symbol from the app to the post so it’s clear it was reposted. Users who want the photo to appear without the watermark should probably stick to screenshotting the post and then posting it.