diff --git a/lib/types/minimally-encoded-thread-permissions-types.js b/lib/types/minimally-encoded-thread-permissions-types.js
--- a/lib/types/minimally-encoded-thread-permissions-types.js
+++ b/lib/types/minimally-encoded-thread-permissions-types.js
@@ -26,11 +26,17 @@
 
 const minimallyEncodeRoleInfo = (
   roleInfo: LegacyRoleInfo,
-): MinimallyEncodedRoleInfo => ({
-  ...roleInfo,
-  minimallyEncoded: true,
-  permissions: threadRolePermissionsBlobToBitmaskArray(roleInfo.permissions),
-});
+): MinimallyEncodedRoleInfo => {
+  invariant(
+    !('minimallyEncoded' in roleInfo),
+    'roleInfo is already minimally encoded.',
+  );
+  return {
+    ...roleInfo,
+    minimallyEncoded: true,
+    permissions: threadRolePermissionsBlobToBitmaskArray(roleInfo.permissions),
+  };
+};
 
 const decodeMinimallyEncodedRoleInfo = (
   minimallyEncodedRoleInfo: MinimallyEncodedRoleInfo,
@@ -84,11 +90,17 @@
 
 const minimallyEncodeMemberInfo = (
   memberInfo: LegacyMemberInfo,
-): MinimallyEncodedMemberInfo => ({
-  ...memberInfo,
-  minimallyEncoded: true,
-  permissions: permissionsToBitmaskHex(memberInfo.permissions),
-});
+): MinimallyEncodedMemberInfo => {
+  invariant(
+    !('minimallyEncoded' in memberInfo),
+    'memberInfo is already minimally encoded.',
+  );
+  return {
+    ...memberInfo,
+    minimallyEncoded: true,
+    permissions: permissionsToBitmaskHex(memberInfo.permissions),
+  };
+};
 
 const decodeMinimallyEncodedMemberInfo = (
   minimallyEncodedMemberInfo: MinimallyEncodedMemberInfo,
@@ -110,11 +122,17 @@
 
 const minimallyEncodeRelativeMemberInfo = (
   relativeMemberInfo: LegacyRelativeMemberInfo,
-): MinimallyEncodedRelativeMemberInfo => ({
-  ...relativeMemberInfo,
-  minimallyEncoded: true,
-  permissions: permissionsToBitmaskHex(relativeMemberInfo.permissions),
-});
+): MinimallyEncodedRelativeMemberInfo => {
+  invariant(
+    !('minimallyEncoded' in relativeMemberInfo),
+    'relativeMemberInfo is already minimally encoded.',
+  );
+  return {
+    ...relativeMemberInfo,
+    minimallyEncoded: true,
+    permissions: permissionsToBitmaskHex(relativeMemberInfo.permissions),
+  };
+};
 
 const decodeMinimallyEncodedRelativeMemberInfo = (
   minimallyEncodedRelativeMemberInfo: MinimallyEncodedRelativeMemberInfo,
@@ -139,6 +157,10 @@
 const minimallyEncodeRawThreadInfo = (
   rawThreadInfo: LegacyRawThreadInfo,
 ): MinimallyEncodedRawThreadInfo => {
+  invariant(
+    !('minimallyEncoded' in rawThreadInfo),
+    'rawThreadInfo is already minimally encoded.',
+  );
   const { members, roles, currentUser, ...rest } = rawThreadInfo;
   return {
     ...rest,
@@ -173,6 +195,10 @@
 const minimallyEncodeThreadInfo = (
   threadInfo: LegacyThreadInfo,
 ): MinimallyEncodedThreadInfo => {
+  invariant(
+    !('minimallyEncoded' in threadInfo),
+    'threadInfo is already minimally encoded.',
+  );
   const { members, roles, currentUser, ...rest } = threadInfo;
   return {
     ...rest,