user4220: The minting guide say that you should set the policy to expire after certain slot to make sure that nobody can mint more of the same NFT. This kind of makes sense, but then how can you reuse the same policy to mint more of different NFTs?
I saw that most NFT projects share same policy ID, which makes perfect sense, you can verify that NFT really belongs to that same project and isn't forgery, but does it mean these projects didn't set expiry in their policy? Because if they did, how can they keep minting more and more NFTs over time and add them to their collection? What prevents them from adding two identical NFTs?

user5167: There are a couple different ways minters are doing this. First is they create a policy ID that expires 6-12 months in the future. This allows them to mint multiple collections under the same ID but the ID DOES eventually lock. As long as the ID locks at some point in the future, Pool.pm shows a lock icon.
Other artists choose to permanently have their ID unlocked. This allows them to mint collections under the same ID anytime in the future. As you noted, this makes it somewhat simpler for buyers to associate NFTs with the single ID, however, secondary marketplaces these days make it pretty straight forward to confirm NFTs are authentic even with multiple IDs so it's not a huge advantage.
The other reason an artist might choose to keep the ID open is if the NFT has future utility, such as workbench functionality, or some other feature where the artist might want to CHANGE the actual NFT that's in your wallet.

user6464: NFT projects tend to use a PolicyId to group NFTs into collections.
A PolicyId is tied to a minting wallet and may optionally contain an expiry date.
Once the expiry date passes, the minting wallet cannot mint or update NFTs tied to the PolicyId.
This gives buyers the assurance that a Collection with a expired policyId has not minted more than the stated supply and cannot change the minted asset's metadata.

The minting wallet can Mint two or more identical NFTs, but the NFTs will not technically be a NFT anymore (it becomes fungible).
NFT asset = PolicyId + AssetName
(If 2 Assets in the same PolicyId have the same AssetName, the block explorer will show a count of 2).
NFT buyers will usually check the total supply of NFTs in a PolicyId and that each NFT asset in that PolicyId is unique (non-fungible).

user5122: You are right, some people consider a NFT being a real NFT only after the policy expires, so that you can be sure it is unique.
You can also mint multiple NFTs with one policy in one transaction.

4220: <p>The minting guide say that you should set the policy to expire after certain slot to make sure that nobody can mint more of the same NFT. This kind of makes sense, but then how can you reuse the same policy to mint more of different NFTs?</p>
<p>I saw that most NFT projects share same policy ID, which makes perfect sense, you can verify that NFT really belongs to that same project and isn't forgery, but does it mean these projects didn't set expiry in their policy? Because if they did, how can they keep minting more and more NFTs over time and add them to their collection? What prevents them from adding two identical NFTs?</p>
 5167: <p>There are a couple different ways minters are doing this. First is they create a policy ID that expires 6-12 months in the future. This allows them to mint multiple collections under the same ID but the ID DOES eventually lock. As long as the ID locks at some point in the future, Pool.pm shows a lock icon.</p>
<p>Other artists choose to permanently have their ID unlocked. This allows them to mint collections under the same ID anytime in the future. As you noted, this makes it somewhat simpler for buyers to associate NFTs with the single ID, however, secondary marketplaces these days make it pretty straight forward to confirm NFTs are authentic even with multiple IDs so it's not a huge advantage.</p>
<p>The other reason an artist might choose to keep the ID open is if the NFT has future utility, such as workbench functionality, or some other feature where the artist might want to CHANGE the actual NFT that's in your wallet.</p>
 6464: <p>NFT projects tend to use a PolicyId to group NFTs into collections.</p>
<p>A PolicyId is tied to a minting wallet and may optionally contain an expiry date.</p>
<p>Once the expiry date passes, the minting wallet cannot mint or update NFTs tied to the PolicyId.</p>
<p>This gives buyers the assurance that a Collection with a expired policyId has not minted more than the stated supply and cannot change the minted asset's metadata.</p>
<hr />
<p>The minting wallet can Mint two or more identical NFTs, but the NFTs will not technically be a NFT anymore (it becomes fungible).</p>
<p>NFT asset = PolicyId + AssetName</p>
<p>(If 2 Assets in the same PolicyId have the same AssetName, the block explorer will show a count of 2).</p>
<p>NFT buyers will usually check the total supply of NFTs in a PolicyId and that each NFT asset in that PolicyId is unique (non-fungible).</p>
 5122: <p>You are right, some people consider a NFT being a real NFT only after the policy expires, so that you can be sure it is unique.
You can also mint multiple NFTs with one policy in one transaction.</p>
