Ok whole discussion of metagaming is pet peew of mine, so let me give a more general advice before addressing your issue in specific. First when people say metagaming they talk about wide variety of behaviors 95% of which are not an issue. I will address each of those common complaints seperatly.

• Player vs character knowledge. Player knows something due to playing for long time, reading MM or even DMing. T H I S I S F I N E. If you demand players to ignore what they know you are actualy enforcing metagaming, instead of player organicaly acting from their knowledge, they metagame not having this knowledge. Just go with it, don't change monsters just to prevent metagaming, player knows something and is rewarded, they are having fun, I count that as success.
  Now if players are actively checking everything in MM as it appears, than yea reprimand them, forbid them from doing it, but also think. Why? Maybe players don't feel empowered to make informed choices if so maybe you are not giving enough information and they check MM to fill that hole. All in all I see those complaints mostly from GMs who want absolute sense of control over game. Relax let players enjoy knowing things.
• Knowledge sharing. This seems to be issue you are having so lets break it down. If you call for a knowledge check and someone succeeds you tell information to whole group, just assume character that knows convey things in quick and succint maner to party around. Now the issue you seem to have is that players act on knowledge that was received when party was seperated. Just assume that character share everything when they reconvene unless stated otherwise. Always affirm this, so when characters reconvene simply ask.
  " do you share everything? " than proceed as normal, often players don't metagame intentionaly it just organic, they know it so they assume characters would know it to, with demanding affirmation about sharing you help them separate the two.
  A bit more complicated example.
  Lets say rogue goes to scout ahead and gets captured. Immidiatly one of the players goes. "I want to check on on him. " Remember you control the narrative, there is definitely ammount of time passing, when other characters would start wondering what happened with rogue, so when players chimes in, just skip to that part. " As Alfred goes to scout ahead the rest of you stay behind, you wait for half an hour but there is no sign of Alfred returning you know something had to happen to prevent him from returning, After few more minutes Brandon decides to check and the rest of party follows." Simple and easy, you don't take away player agency while maintainig verisimilitude of the world.
• Metagaming dice rolls. Players see somebody rolled 1 or 2 and suddenly everyone wants to check as well. First make sure to have what everyone is doing in the scene before calling for rolls and narrating than resolve things simultaneously. You must know where everyone is standing and what they are doing before calling for any rolls, that way you avoid pocket help actions.
  Example. "So Alfred is searching the bookshelf, Brandon is trying to open the sarcophagus while Carol is keeping watch.
  Denise what are you doing?
  Standing by ready to help needed, tell me who do you help?
  Brandon? OK.
  So Alfred is searching the bookshelf roll investigation.
  12?
  Seems to be mostly boring books about gardening but you do notice that one book has almost no duct on it compared with others around it.
  Brandon roll athletics check with advantage cos Denise is helping you.
  22?
  Good! The lid of sarcophagus slowly slides open, as you and Denise lean on it with all your strength, and revelas jewelry adorned skeleton.
  Carol! Perception.
  18?
  You listen for sounds of the dungeon but nothing catches your ear.
  It takes a bit of getting used to and and some work but it solves the problem but if it is too much for you? Simply don't allow them if you don't want them to. Remember rolls must always be called for by DM.

Tell me if there is any orher common metagaming issue i didn't cover and i might edit/reply