diff --git a/spec/FieldsArray.ts b/spec/FieldsArray.ts
index 00bd2c0..2475108 100644
--- a/spec/FieldsArray.ts
+++ b/spec/FieldsArray.ts
@@ -108,5 +108,88 @@ describe("FieldsArray", () => {
 			expect(pool.has("t1")).toBe(false);
 			expect(pool.has("k1")).toBe(true);
 		});
+
+		it("should log a warning if key already exists and omit that object", () => {
+			fa.push({ key: "t2", value: "v2" });
+			fa.push({ key: "t3", value: "v3" });
+			fa.push({ key: "t4", value: "v4" });
+
+			console.warn = jasmine.createSpy("log");
+
+			fa.splice(0, 1, { key: "t2", value: "v2" });
+
+			expect(console.warn).toHaveBeenCalledWith(
+				Messages.FIELDS.REPEATED_KEY.replace("%s", "t2"),
+			);
+
+			expect(fa.length).toBe(3);
+		});
+	});
+
+	describe("shift", () => {
+		beforeEach(() => {
+			fa.push({ key: "t1", value: "v1" });
+			fa.push({ key: "t2", value: "v2" });
+		});
+
+		it("should prevent popping out fields if pass is frozen", () => {
+			frozen = true;
+
+			expect(() => fa.shift()).toThrowError(
+				Error,
+				Messages.BUNDLE.CLOSED,
+			);
+		});
+
+		it("should shift out fields", () => {
+			expect(fa.shift()).toEqual({ key: "t1", value: "v1" });
+			expect(fa.length).toBe(1);
+			expect(fa[0]).toEqual({ key: "t2", value: "v2" });
+		});
+
+		it("should remove the key from the pool", () => {
+			expect(pool.has("t1")).toBe(true);
+
+			fa.shift();
+
+			expect(pool.has("t1")).toBe(false);
+		});
+	});
+
+	describe("unshift", () => {
+		it("should prevent adding new fields if pass is frozen", () => {
+			frozen = true;
+
+			expect(() => fa.unshift({ key: "t1", value: "v1" })).toThrowError(
+				Error,
+				Messages.BUNDLE.CLOSED,
+			);
+		});
+
+		it("should allow adding fields", () => {
+			expect(fa.unshift({ key: "t1", value: "v1" })).toBe(1);
+			expect(fa.length).toBe(1);
+			expect(fa[0]).toEqual({ key: "t1", value: "v1" });
+		});
+
+		it("should add the key to the pool", () => {
+			fa.unshift({ key: "t1", value: "v1" });
+
+			expect(pool.has("t1")).toBe(true);
+		});
+
+		it("should log a warning if key already exists and omit that object", () => {
+			fa.push({ key: "t1", value: "v1" });
+
+			console.warn = jasmine.createSpy("log");
+
+			fa.unshift({ key: "t1", value: "v1" });
+
+			expect(console.warn).toHaveBeenCalledWith(
+				Messages.FIELDS.REPEATED_KEY.replace("%s", "t1"),
+			);
+
+			expect(fa.length).toBe(1);
+		});
 	});
 });