nix-profile*.sh.in: append PATHs idempotently

When people source the Nix profile scripts more than once,
they can cause appended PATH variables to grow longer each
time. This is somewhat benign, but it does spook people and
cause support load.

scripts/nix-profile-daemon.sh.in

@@ -1,6 +1,6 @@
 # Only execute this file once per shell.
 if [ -n "${__ETC_PROFILE_NIX_SOURCED:-}" ]; then return; fi
-__ETC_PROFILE_NIX_SOURCED=1
+export __ETC_PROFILE_NIX_SOURCED=1
 
 NIX_LINK=$HOME/.nix-profile
 if [ -n "${XDG_STATE_HOME-}" ]; then
@@ -60,5 +60,12 @@ else
   unset -f check_nix_profiles
 fi
 
-export PATH="$NIX_LINK/bin:@localstatedir@/nix/profiles/default/bin:$PATH"
+# only append once
+case ":$PATH:" in
+  *:"$NIX_LINK/bin:@localstatedir@/nix/profiles/default/bin":*)
+    ;;
+  *)
+    export PATH="$NIX_LINK/bin:@localstatedir@/nix/profiles/default/bin:$PATH"
+    ;;
+esac
 unset NIX_LINK

scripts/nix-profile.sh.in

@@ -1,3 +1,7 @@
+# Only execute this file once per shell.
+if [ -n "${__ETC_PROFILE_NIX_SOURCED:-}" ]; then return; fi
+export __ETC_PROFILE_NIX_SOURCED=1
+
 if [ -n "$HOME" ] && [ -n "$USER" ]; then
 
     # Set up the per-user profile.
@@ -51,9 +55,24 @@ if [ -n "$HOME" ] && [ -n "$USER" ]; then
     # pick up `.nix-profile/share/man` because is it close to `.nix-profile/bin`
     # which is in the $PATH. For more info, run `manpath -d`.
     if [ -n "${MANPATH-}" ]; then
-        export MANPATH="$NIX_LINK/share/man:$MANPATH"
+        # only append once
+        case ":$MANPATH:" in
+          *:"$NIX_LINK/share/man":*)
+            ;;
+          *)
+            export MANPATH="$NIX_LINK/share/man:$MANPATH"
+            ;;
+        esac
     fi
 
-    export PATH="$NIX_LINK/bin:$PATH"
+    # only append once
+    case ":$PATH:" in
+      *:"$NIX_LINK/bin":*)
+        ;;
+      *)
+        export PATH="$NIX_LINK/bin:$PATH"
+        ;;
+    esac
+
     unset NIX_LINK NIX_LINK_NEW
 fi